111. Minimum Depth of Binary Tree

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.

求根节点到叶子节点的距离，要求输出最小值

C++（13ms）：

/**
 * 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 ;
        queue<TreeNode*> que ;
        que.push(root) ;
        int dep = 0 ;
        while(!que.empty()){
            int len = que.size() ;
            dep++ ;
            for(int i = 0 ; i < len ; i++){
                TreeNode* node = que.front() ;
                que.pop() ;
                if (node->left == NULL && node->right == NULL) 
                    return dep ;
                if (node->left != NULL) 
                    que.push(node->left) ;
                if (node->right != NULL) 
                    que.push(node->right) ;
            }
        }
    }
};