树01

二叉树的递归遍历

前序

/**
 * 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:
    void traversal(TreeNode *node, vector<int> &result)
    {
        if(node == nullptr)
            return;
        result.push_back(node->val);
        traversal(node->left, result);
        traversal(node->right, result);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        if(root == nullptr) return result;
        traversal(root, result);
        return result;
    }
};

中序

/**
 * 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:
    void traversal(TreeNode *node, vector<int> &result)
        {
            if(node == nullptr)
                return;
           
            traversal(node->left, result);
            result.push_back(node->val);
            traversal(node->right, result);
        }
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> result;
        if(root == nullptr) return result;
        traversal(root, result);
        return result;
    }
};

后序

/**
 * 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:
    void traversal(TreeNode *node, vector<int> &result)
        {
            if(node == nullptr)
                return;
            traversal(node->left, result);
            traversal(node->right, result);
            result.push_back(node->val);
        }
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> result;
        if(root == nullptr) return result;
        traversal(root, result);
        return result;
    }
};

二叉树迭代遍历

前序

/**
 * 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> preorderTraversal(TreeNode* root) {
        vector<int> result;
        if(root == nullptr) return result;
        stack<TreeNode*> st;
        st.push(root);
        while(!st.empty())
        {
            TreeNode *cur = st.top();
            st.pop();
            if(cur == nullptr) continue;
            else
            {
                result.push_back(cur->val);
                st.push(cur->right);
                st.push(cur->left);
            }
        }
        return result;
    }
};

中序

/**
 * 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> inorderTraversal(TreeNode* root) {
        vector<int> result;
        if(root == nullptr) return result;
        stack<TreeNode*> st;
        TreeNode *cur = root;
        while(cur != nullptr || !st.empty())
        {
            if(cur != nullptr) 
            {st.push(cur);
            cur = cur -> left;
            }
            else
            {
                cur = st.top();
                st.pop();
                result.push_back(cur->val);
                cur = cur->right;
            }
        }
        return result;
    }
};

后序

/**
 * 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> postorderTraversal(TreeNode* root) {
        vector<int> result;
        if(root == nullptr) return result;
        stack<TreeNode*> st;
        st.push(root);
        while(!st.empty())
        {
            TreeNode *cur = st.top();
            st.pop();
            if(cur == nullptr) continue;
            else
            {
                result.push_back(cur->val);
                st.push(cur->left);
                st.push(cur->right);
            }
        }
        reverse(result.begin(), result.end());
        return result;
    }
};

层次遍历

/**
 * 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) {
        queue<TreeNode*> que;
        vector<vector<int>> results;
        TreeNode *cur = root;
        if(cur == nullptr)  return results;
        que.push(cur);
        while(!que.empty())
        {
            int size = que.size();
            vector<int> result;
            while(size--)
            {
                cur = que.front();
                que.pop();
                result.push_back(cur->val);
                if(cur->left) que.push(cur->left);
                if(cur->right) que.push(cur->right);
            }
            results.push_back(result);
        }
        return results;
    }
};