leetcode 111: Minimum Depth of Binary Tree

問題描述：

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：

思路很簡單，就是直接地按層遍歷二叉樹，然後發現葉節點就跳出即可。
但是一開始做得很糊塗，以爲一個節點沒有兩個兒子就當做葉節點，折騰了好久才發現錯誤，而且這個錯誤在做Path Sum那題的時候也有這麼一個折騰的過程，但當時沒有記下，反而讓錯誤思維固勢。特此記錄。

代碼：

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;
    }

};