[LeetCode] Minimum Depth of Binary Tree

题目：

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

解答：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int minDepth(TreeNode *root) {
        if(root == NULL) {
            return 0;
        }
        if(root->left == NULL) {
            return 1 + minDepth(root->right);
        }
        if(root->right == NULL) {
            return 1 + minDepth(root->left);
        }
        return 1 + min(minDepth(root->left), minDepth(root->right));
    }
};

思路：

和“Maximum Depth of Binary Tree”有区别，求min时如果某一个分支为NULL，这个分支是忽略不计的，不能算成1+min(有分支的min，0)