Leetcode: Minimum Depth of Binary Tree

题目：
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.

思路分析：
求二叉树的最小深度。二叉树多用迭代。

C++示例代码：

/**
 * Definition for binary tree
 * 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;
        }

        //leftDepth=0,rightDepth=0直接返回1
        if (root->left == NULL && root->right == NULL)
        {
            return 1;
        }

        int leftDepth = minDepth(root->left);
        int rightDepth = minDepth(root->right);

        //这里leftDepth和rightDepth不可能同时为0
        if (leftDepth == 0)
        {
            return rightDepth + 1;
        }
        else if (rightDepth == 0)
        {
            return leftDepth + 1;
        }
        else
        {
            return min(leftDepth, rightDepth) + 1;
        }
    }
};