leetcode 111: Minimum Depth of Binary Tree

问题描述：

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.

思路：

思路很简单，就是直接地按层遍历二叉树，然后发现叶节点就跳出即可。
但是一开始做得很糊涂，以为一个节点没有两个儿子就当做叶节点，折腾了好久才发现错误，而且这个错误在做Path Sum那题的时候也有这么一个折腾的过程，但当时没有记下，反而让错误思维固势。特此记录。

代码：

class Solution {   
public:
    int minDepth(TreeNode* root) {
        list<TreeNode*> nodelist;
        if (root == NULL) return 0;
        nodelist.push_back(root);

        int depth = 0;
        bool isbreak = false;    //is get from break
        while ((!nodelist.empty()) && (!isbreak))
        {
            int length = nodelist.size();
            for (int i = 0; i < length; i++)
            {
                TreeNode *now = *nodelist.begin();
                if ((now->left == NULL) && (now->right == NULL))     //determine if it is a leaf
                {
                    isbreak = true;   //find the leaf, set break signal
                    break;
                }
                if (now->left) nodelist.push_back(now->left);
                if (now->right) nodelist.push_back(now->right);
                nodelist.pop_front();
            }
            depth++;
        }
        return depth;
    }

};