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.

1. different from maximum depth. if a node only has one child, the depth will be 1 + child depth.

/**

* 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) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        return minRec(root);    }        int minRec( TreeNode * root) {        if(!root) return 0;                int left = minRec( root->left);        int right = minRec( root->right);                if(left && right) return 1 + min(left, right);        if(left || right) return 1+left+right;        return 1;    }};