Leetcode NO.111 Minimum Depth of Binary Tree

本题题目要求如下：

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.
这道题是一道很简单的题目，基本思想就是DFS，但是DFS要多一个参数来测定该层的层数，再定义一个全局变量来表示此时的最小层数，并不断更新它即可，代码如下所示：

/**
 * Definition for a binary tree node.
 * 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 == nullptr) {
            return 0;
        }
        dfs(root, 1);
        return globalMin;
    }
private:
    int globalMin = INT_MAX;
    void dfs(TreeNode* root, int level) {
        if (level >= globalMin) {
            return;
        }
        if (root->left == nullptr and root->right == nullptr) {
            globalMin = level;
        }
        if (root->left != nullptr) {
            dfs(root->left, level+1);
        }
        if (root->right != nullptr) {
            dfs(root->right, level+1);
        }
    }
};