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Maximum Depth of Binary Tree

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

Example

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3

Thoughts

  1. The depth of a node is the maximum depth of its left depth and right depth plus one.
  2. Empty node has zero depth.

Code

/**
 * 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) {
        return depth(root);
    }
    int depth(TreeNode* node){
        if(node == nullptr) {
            return 0;
        }
        auto leftDepth = depth(node->left);
        auto rightDepth = depth(node->right);
        return max(leftDepth, rightDepth) + 1;
    }
};

References: https://leetcode-cn.com/problems/maximum-depth-of-binary-tree/