107. Binary Tree Level Order Traversal II

最新推荐文章于 2021-08-10 13:02:56 发布

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最新推荐文章于 2021-08-10 13:02:56 发布

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分类专栏： leetcode

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本文介绍了一种算法，用于实现二叉树的从叶节点到根节点的层次遍历，并返回每一层节点的值。该算法使用了队列和栈的数据结构来辅助遍历过程。

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Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its bottom-up level order traversal as:

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

/**
 * 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:
	vector<vector<int>> levelOrderBottom(TreeNode* root) {
		vector<vector<int> >res;
		if (root == NULL){
			return res;
		}

		stack<vector<int> >stk;
		queue<TreeNode*> Q;
		Q.push(root);
		Q.push(NULL);
		vector<int> level;
		while (!Q.empty()){
			TreeNode* front = Q.front();
			Q.pop();
			if (front){
				level.push_back(front->val);
				if (front->left){
					Q.push(front->left);
				}
				if (front->right){
					Q.push(front->right);
				}
			}
			else{
			    stk.push(level);
				level.clear();
				if (!Q.empty()){//
					Q.push(NULL);
				}
			}
		}
		while (!stk.empty()){
			res.push_back(stk.top());
			stk.pop();
		}
		return res;
	}
};