二叉树

1. 二叉树的前序遍历

• 先访问根节点再访问左子树和右子树
• 采用递归的方式遍历所有结点

/**
 * 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 {
    vector<int> res;
    void dfs(TreeNode* root) {
        if(root) {
            res.push_back(root->val);
            dfs(root->left);
            dfs(root->right);
        }
    }
public:
    vector<int> preorderTraversal(TreeNode* root) {
        dfs(root);
        return res;

    }
};

2. 二叉树的中序遍历

• 先访问二叉树的左子树再访问结点最后访问右子树
• 采用递归的方式遍历所有结点

/**
 * 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 {
    vector<int> res;
    void dfs(TreeNode* root) {
        if(root) {
            dfs(root->left);
            res.push_back(root->val);
            dfs(root->right);
        }
    }
public:
    vector<int> inorderTraversal(TreeNode* root) {
       res.clear();
       dfs(root);
       return res;
    }
};

3. 二叉树的后序遍历

• 先遍历左子树，再遍历右子树，最后遍历根结点
• 还是递归

/**
 * 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 {
    vector<int> res;
    void dfs(TreeNode* root) {
        if(root) {
            dfs(root->left);
            dfs(root->right);
            res.push_back(root->val);
        }
    }
public:
    vector<int> postorderTraversal(TreeNode* root) {
        res.clear();
        dfs(root);
        return res;
    }
};

4. 二叉树的最大深度

• 二叉树的最大深度就是左子树或者右子树的最大深度+1
• 所以先计算出左子树的深度
• 再计算出右子树的深度
• 方法是递归

/**
 * 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) {
        if(root == nullptr) return 0;
        return max(maxDepth(root->left), maxDepth(root->right)) +1;
    }
};