LeetCode-111. Minimum Depth of Binary Tree

問題：
https://leetcode.com/problems/minimum-depth-of-binary-tree/?tab=Description
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.
給定一個二叉樹，找出它的最短深度。最短深度是指從節點到最近的葉節點的最短距離
分析：
分幾種情況討論。root爲空，root左節點或者右節點爲空，root左右節點都不爲空。
參考C++代碼：

/**
 * 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 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);
        int l=minDepth(root->left);
        int r=minDepth(root->right);
        return l<r?l+1:r+1;
    }
};