104、Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

思路

求 root 的最大深度，那么我就需要求出 left 的最大深度，以及 right 的最大深度。这是个递归问题。

c++11

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if (!root) return 0;
        return std::max(maxDepth(root->left), maxDepth(root->right)) + 1;
    }
};

ac结果：

Runtime: 16 ms, faster than 99.11% of C++ online submissions for Maximum Depth of Binary Tree.
Memory Usage: 19.4 MB, less than 55.13% of C++ online submissions for Maximum Depth of Binary Tree.