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.

思考：考虑[1,2]这种情况，返回树最低高度为2。所以分两种情况，有一棵子树不在时，返回另一棵子树树高+1，否则返回矮子树高+1.

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {private:    int ret;public:    int DFS(TreeNode *root)    {        if(root==NULL) return 0;        else if(!root->left&&!root->right) return 1;        int height1=DFS(root->left);        int height2=DFS(root->right);        if(height1==0) return height2+1;        if(height2==0) return height1+1;        return min(height1,height2)+1;    }    int minDepth(TreeNode *root) {        if(root==NULL) return 0;        ret=DFS(root);        return ret;    }};