104. Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2:

Input: root = [1,null,2]
Output: 2

Example 3:

Input: root = []
Output: 0

Example 4:

Input: root = [0]
Output: 1

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

解題技巧:

recursive終止條件

1. 遇到root == NULL : 當樹是空樹時，return 0
2. root是leaf node : return 1

最後return左子樹和右子樹中深度較深的樹，+1為root

Runtime: 8 ms, faster than 64.75% of C++ online submissions for Maximum Depth of Binary Tree.

Memory Usage: 18.8 MB, less than 51.18% of C++ online submissions for Maximum Depth of Binary Tree.