学步园

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：

首先要区分两种结点：非leaf的结点和leaf的结点。leaf结点的左右子树均为空。

然后对树进行深度优先搜索的同时记录树的当前深度，发现leaf结点时将深度和已知的最小深度比较。

题解：

/**
 * 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 min_depth;
    
    void minDepthProc(TreeNode* node, int depth)
    {
        if (node == nullptr)
            return;
        
        if (node->left == nullptr && node->right == nullptr)
        {
            // this is a leaf node
            if (depth < min_depth)
                min_depth = depth;
            return;
        }
        
        minDepthProc(node->left, depth + 1);
        minDepthProc(node->right, depth + 1);
    }
    
    int minDepth(TreeNode *root) {
        if (root == nullptr)
            return 0;
        
        min_depth = 9999999;    // should be std::numeric_limits<int>::max();
        minDepthProc(root, 1);
        
        return min_depth;
    }
};