Minimum Depth of Binary Tree(二叉树深度)

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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void dfs(TreeNode *root, int &Min, int step){
        if(root == NULL) return;
        if(root->left == NULL && root->right == NULL){
            if(Min>step) Min=step;
            return;
        }
        dfs(root->left, Min, step+1);
        dfs(root->right, Min, step+1);
    }
    int minDepth(TreeNode *root) {
        if(root == NULL) return 0;
        int Min = 999999;
        dfs(root, Min, 1);
        return Min;
    }
};