104. Maximum Depth of Binary Tree

題目原文

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.

解題思路

1. 參考下圖。

          3
         (3)
        /   \
       /     \
      /       \
     /         \
    9           20
   (1)          (2)
  /   \        /   \
(0)   (0)     /     \
            15       7      
           (1)       (1) 
          /   \     /   \
        (0)   (0)  (0)  (0)
      
      

程式解答

/**
 * 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 max(1 + maxDepth(root->left), 1 + maxDepth(root->right));
    }
};