leetcode 107 Binary Tree Level Order Traversal II

最新推荐文章于 2019-05-09 23:27:00 发布

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本文介绍了一种算法，用于实现二叉树的层次遍历并以从叶节点到根节点的顺序返回结果。通过使用队列来跟踪节点，该算法能够有效地按层次收集节点值，并逆序输出。

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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]
]

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/**
 * 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;
		vector<int> vec;

		if(root==NULL) return res;

		queue<TreeNode*> q;

		q.push(root);
		q.push(NULL);

		while(!q.empty()) {
			TreeNode *temp = q.front();
			q.pop();

			if(temp!=NULL) {
				vec.push_back(temp->val);
				if(temp->left!=NULL)
					q.push(temp->left);
				if(temp->right!=NULL)
					q.push(temp->right); 
			} else {
				res.push_back(vec);
				vec.clear();
				q.push(NULL);
				if(q.front()==NULL) break;
			}
		}

		reverse(res.begin(), res.end());

		return res;
    }
};