leetcode - Binary Tree Level Order Traversal

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Given a binary tree, return the level order traversal of its nodes‘ values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   /   9  20
    /     15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

confused what "{1,#,2,3}" means?

struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    std::vector<std::vector<int> > levelOrder(TreeNode *root){
        //第一种解法，利用队列BFS分层遍历二叉树
		std::vector<std::vector<int>> vec;
        if(root == NULL) return vec;
		std::queue<TreeNode*> Q;
		Q.push(root);
		int cnt = 1;
		std::vector<int> tmp;
		while(!Q.empty())
		{
			tmp.clear();
			int level = 0;
			for(int i = 0; i < cnt; i++)
			{
				root = Q.front();
				Q.pop();
				tmp.push_back(root->val);
				if(root->left != NULL) Q.push(root->left),level++;
				if(root->right != NULL) Q.push(root->right),level++;
			}
			cnt = level;
			vec.push_back(tmp);
		}
		return vec;
    }
};

class Solution {
public:
    std::vector<std::vector<int> > levelOrder(TreeNode *root){
        //第二种方法也是利用队列的方法来做的，但是，其实数据结构的实现是vector,觉得这种方法更简便一些.
		std::vector<std::vector<int>> result;
		if(root == NULL) return result;
		std::vector<int> vec;
		std::vector<TreeNode*> node;
		node.push_back(root);
		int cur = 0,last = 1;
		while(cur < node.size())
		{
			last = node.size();
			while(cur < last)
			{
				vec.push_back(node[cur]->val);
				if(node[cur]->left != NULL) node.push_back(node[cur]->left);
				if(node[cur]->right != NULL) node.push_back(node[cur]->right);
				cur++;
			}
			result.push_back(vec);
			vec.clear();
		}
		return result;
    }
};

class Solution {
public:
    std::vector<std::vector<int> > levelOrder(TreeNode *root){
        //第三种解法，dfs分层遍历二叉树
		dfs(root,0);
		return result;
    }
private:
	std::vector<std::vector<int>> result;
	void dfs(TreeNode *root,int level)
	{
		if(root == NULL) return;
		if(level == result.size())
		{
			std::vector<int> vec;
			result.push_back(vec);
		}
		result[level].push_back(root->val);
		dfs(root->left,level+1);
		dfs(root->right,level+1);
	}
};

leetcode - Binary Tree Level Order Traversal

标签：des   style   color   io   os   ar   for   数据   sp   

原文地址：http://blog.csdn.net/akibatakuya/article/details/39672355