#155 Minimum Depth of Binary Tree

题目描述：

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.

Have you met this question in a real interview? 

Yes

Example

Given a binary tree as follow:

  1 / \ 2   3   / \  4   5

The minimum depth is 2.

题目思路：

这题我还是用了bfs比较方便，用一个queue来记录从top到bottom的nodes，再用一个queue来记录相应nodes的level。当发现有node没有子树的时候，它的level就是我们所在的答案。

Mycode（AC = 76ms）：

/** * Definition of TreeNode: * class TreeNode { * public: *     int val; *     TreeNode *left, *right; *     TreeNode(int val) { *         this->val = val; *         this->left = this->right = NULL; *     } * } */class Solution {public:    /**     * @param root: The root of binary tree.     * @return: An integer     */    int minDepth(TreeNode *root) {        // write your code here        if (root == NULL) {            return 0;        }                // store the nodes from top to bottom        queue<TreeNode *> helper;        helper.push(root);        // store the level of each node        queue<int> levels;        levels.push(1);                int level;        while (!helper.empty()) {            // get the node and level from front of queue            TreeNode *node = helper.front();            helper.pop();            level = levels.front();            levels.pop();                        // if it is single node, then return level            if (!node->left && !node->right) {                return level;            }                        // push the left/right node into queue            if (node->left) {                helper.push(node->left);                levels.push(level + 1);            }            if (node->right) {                helper.push(node->right);                levels.push(level + 1);            }        }                return level;            }};