给定二叉树 [3,9,20,null,null,15,7],
​
    3
   / \
  9  20
    /  \
   15   7
返回它的最小深度  2.

/**
 * 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) return 0;
        if(!root->left && !root->right) return 1;
        if(!root->left) return 1+minDepth(root->right);
        if(!root->right) return 1+minDepth(root->left);
        return min(minDepth(root->left)+1,minDepth(root->right)+1);
​
    }
};