LeetCode Minimum Depth of Binary Tree 最小深度二叉树

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.

本题思路：

1. 每进入一层用一个变量（这里是levelNum）记录当前从根节点到这一层节点的节点数

2. 当到达叶子节点的时候，就记录当前最小深度overall

2. 当所有的叶子节点都到达一遍之后，就得到了最终最小深度overall。

class Solution {
public:
	int minDepth(TreeNode *root) {
		if(root == nullptr) return 0;
		int overall = INT_MAX;
		int levelNum = 0;
		minD(root, overall, levelNum);
		return overall;
	}

	void minD(TreeNode *node, int &overall, int levelNum)
	{
		if(node == nullptr) return;
		levelNum++;
		minD(node->left, overall, levelNum);
		minD(node->right, overall, levelNum);
		if(node->left==nullptr && node->right==nullptr)
			overall = min(overall,levelNum);
	}
};

//2014-2-16 update
	int minDepth(TreeNode *root) 
	{
		if (!root) return 0;
		if (!root->left && !root->right) return 1;
		int L = root->left? minDepth(root->left)+1:INT_MAX;
		int R = root->right? minDepth(root->right)+1:INT_MAX;
		return min(L, R);
	}