[LeetCode]Binary Tree Level Order Traversal

最新推荐文章于 2021-11-17 20:57:12 发布

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本文介绍了一种使用广度优先搜索（BFS）进行二叉树层次遍历的方法，并提供了详细的C++实现代码。通过引入节点级别的层级信息，该算法能够有效地按层输出二叉树的所有节点值。

struct TreeNode {
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
	//BFS is always work for the level order traverse of a tree
	//all we need is add some additional information, to solve the specific problem of level order type
public:
	vector<vector<int>> LevelOrder_BFS(TreeNode* root)
	{
		vector<vector<int>> ans;
		if(!root)//put the null tree judge here will be more neet
			return ans;
		queue<pair<TreeNode*, int>> q;
		int stepNow = 0;
		q.push(make_pair(root, stepNow));
		vector<int> curLevel;
		while(!q.empty())
		{
			TreeNode* curNode = q.front().first;
			int curStep = q.front().second;
			q.pop();
			
			if(curStep != stepNow)
			{
				ans.push_back(curLevel);
				curLevel.clear();
				stepNow = curStep;
			}
			curLevel.push_back(curNode->val);
			if(curNode->left)
				q.push(make_pair(curNode->left, curStep+1));
			if(curNode->right)
				q.push(make_pair(curNode->right, curStep+1));

		}
		ans.push_back(curLevel);//note: we should remember the last level here
		return ans;
	}
    vector<vector<int> > levelOrder(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
		return LevelOrder_BFS(root);
    }
};

second time

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    struct Node
    {
        TreeNode* treeNode;
        int level;
        Node(TreeNode* _treeNode = NULL, int _level = 0):treeNode(_treeNode), level(_level){};
    };
    vector<vector<int> > levelOrder(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(root == NULL) return vector<vector<int> >();
        queue<Node> nodeQ;
        nodeQ.push(Node(root, 1));
        int prevLevel = 0;
        vector<int> curLevelAns;
        vector<vector<int> > allLevelAns;
        while(!nodeQ.empty())
        {
            TreeNode* curNode = nodeQ.front().treeNode;
            int curLevel = nodeQ.front().level;
            nodeQ.pop();
            if(curNode == NULL) continue;
            if(curLevel != prevLevel)
            {
                if(prevLevel != 0) allLevelAns.push_back(curLevelAns);
                curLevelAns.clear();
            }
            prevLevel = curLevel;
            curLevelAns.push_back(curNode->val);
            nodeQ.push(Node(curNode->left, curLevel+1));
            nodeQ.push(Node(curNode->right, curLevel+1));
        }
        allLevelAns.push_back(curLevelAns);
        
        return allLevelAns;
    }
};