day15 - 二叉树 part02

102. 二叉树的层序遍历

详解

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        /*
        // 没想到用size来固定每轮取的数量，用了个临时队列做中转
        vector<vector<int>> result;
        queue<TreeNode*> queue_1;
        if(root == NULL)
            return result;
        queue_1.push(root);
        while(!queue_1.empty()){
            vector<int> tmp;
            queue<TreeNode*> queue_2;
            while(!queue_1.empty()){
                TreeNode* node = queue_1.front();
                tmp.push_back(node->val);
                if(node->left != NULL) queue_2.push(node->left);
                if(node->right != NULL) queue_2.push(node->right);
                queue_1.pop();
            }
            while(!queue_2.empty()){
                queue_1.push(queue_2.front());
                queue_2.pop();
            }
            result.push_back(tmp);
        }
        */

        queue<TreeNode*> que;
        if (root != NULL) que.push(root);
        vector<vector<int>> result;
        while (!que.empty()) {
            int size = que.size();
            vector<int> vec;
            // 这里一定要使用固定大小size，不要使用que.size()，因为que.size是不断变化的
            for (int i = 0; i < size; i++) {
                TreeNode* node = que.front();
                que.pop();
                vec.push_back(node->val);
                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
            }
            result.push_back(vec);
        }

        return result;
    }
};

 107. 二叉树的层序遍历 II

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> result;
        queue<TreeNode*> queue_1;
        if(root != NULL) queue_1.push(root);
        while(!queue_1.empty()){
            vector<int> tmp;
            int size = queue_1.size();
            for(int i=0; i< size; i++){
                TreeNode* node = queue_1.front();
                tmp.push_back(node->val);
                if(node->left != NULL) queue_1.push(node->left);
                if(node->right != NULL) queue_1.push(node->right);
                queue_1.pop();
            }
            result.push_back(tmp);
        }
        reverse(result.begin(), result.end());
        return result;
    }
};

199. 二叉树的右视图

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        vector<int> result;
        queue<TreeNode*> queue_1;
        if(root != NULL) queue_1.push(root);
        while(!empty(queue_1)){
            int size = queue_1.size();
            int tmp;
            for(int i=0; i<size; i++){
                TreeNode* node = queue_1.front();
                tmp = node->val;
                queue_1.pop();
                if(node->left) queue_1.push(node->left);
                if(node->right) queue_1.push(node->right);
            }
            result.push_back(tmp);
        }
        return result;
    }
};

 

637. 二叉树的层平均值

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<double> averageOfLevels(TreeNode* root) {
        vector<double> result;
        queue<TreeNode*> queue_1;
        if(root != NULL) queue_1.push(root);
        while(!empty(queue_1)){
            int size = queue_1.size();
            double tmp = 0;
            for(int i=0; i<size; i++){
                TreeNode* node = queue_1.front();
                tmp += node->val;
                queue_1.pop();
                if(node->left) queue_1.push(node->left);
                if(node->right) queue_1.push(node->right);
            }
            result.push_back(tmp/size);
        }
        return result;
    }
};

 

429. N 叉树的层序遍历

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
public:
    vector<vector<int>> levelOrder(Node* root) {
        vector<vector<int>> result;
        queue<Node*> queue_1;
        if(root) queue_1.push(root);

        while(!empty(queue_1)){
            int size = queue_1.size();
            vector<int> tmp;
            for(int i=0; i<size; i++){
                Node* node = queue_1.front();
                tmp.push_back(node->val);
                queue_1.pop();
                for(int j=0; j<node->children.size(); j++){
                    queue_1.push(node->children[j]);
                }
            }
            result.push_back(tmp);
        }
        return result;
    }
};

 

515. 在每个树行中找最大值

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> largestValues(TreeNode* root) {
        vector<int> result;
        queue<TreeNode*> queue_1;
        if(root != NULL) queue_1.push(root);
        while(!empty(queue_1)){
            int size = queue_1.size();
            int tmp = INT_MIN;//-INT_MAX;
            for(int i=0; i<size; i++){
                TreeNode* node = queue_1.front();
                tmp = max(tmp, node->val);
                queue_1.pop();
                if(node->left) queue_1.push(node->left);
                if(node->right) queue_1.push(node->right);
            }
            result.push_back(tmp);
        }
        return result;
    }
};

 116. 填充每个节点的下一个右侧节点指针

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root) {
        queue<Node*> queue_1;
        if(root != NULL) queue_1.push(root);
        while(!empty(queue_1)){
            int size = queue_1.size();
            Node* pre = NULL;
            for(int i=0; i<size; i++){
                Node* node = queue_1.front();
                node->next = pre;
                pre = node;
                queue_1.pop();
                if(node->right) queue_1.push(node->right);
                if(node->left) queue_1.push(node->left);
            }
        }
        return root;
    }
};

117. 填充每个节点的下一个右侧节点指针 II

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root) {
        queue<Node*> queue_1;
        if(root != NULL) queue_1.push(root);
        while(!empty(queue_1)){
            int size = queue_1.size();
            Node* pre = NULL;
            for(int i=0; i<size; i++){
                Node* node = queue_1.front();
                node->next = pre;
                pre = node;
                queue_1.pop();
                if(node->right) queue_1.push(node->right);
                if(node->left) queue_1.push(node->left);
            }
        }
        return root;
    }
};

104. 二叉树的最大深度

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        int result = 0;
        queue<TreeNode*> queue_1;
        if(root != NULL) queue_1.push(root);
        while(!empty(queue_1)){
            int size = queue_1.size();
            for(int i=0; i<size; i++){
                TreeNode* node = queue_1.front();
                queue_1.pop();
                if(node->left) queue_1.push(node->left);
                if(node->right) queue_1.push(node->right);
            }
            result++;
        }
        return result;
    }
};

111. 二叉树的最小深度

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int minDepth(TreeNode* root) {
        if (root == NULL) return 0;
        int depth = 0;
        queue<TreeNode*> que;
        que.push(root);
        while(!que.empty()) {
            int size = que.size();
            depth++; // 记录最小深度
            for (int i = 0; i < size; i++) {
                TreeNode* node = que.front();
                que.pop();
                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
                if (!node->left && !node->right) { // 当左右孩子都为空的时候，说明是最低点的一层了，退出
                    return depth;
                }
            }
        }
        return depth;
    }
};

226. 翻转二叉树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        //前序遍历，从顶点开始转换孩子节点
        if(root == nullptr)
            return root;
        swap(root->left, root->right);
        invertTree(root->left);
        invertTree(root->right);
        return root;
    }
};

101. 对称二叉树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    //递归按层次比较左右2棵树(left->左孩子 == right->右孩子， left->右孩子 == right->左孩子)
    bool compare(TreeNode* left, TreeNode* right){
        //NULL 情况，都为NULL或者一个为NULL
        if(left == NULL && right == NULL) return true;
        else if(left == NULL && right != NULL) return false;
        else if(left != NULL && right == NULL) return false;
        else if(left->val != right->val) return false;
        //节点相同，递归
        bool outside = compare(left->left, right->right);
        bool inside = compare(left->right, right->left);
        bool isSame = outside && inside;                    // 左子树：中、 右子树：中 （逻辑处理）
        return isSame;
    }

    bool isSymmetric(TreeNode* root) {
        if (root == NULL) return true;
        return compare(root->left, root->right);
    }
};