BFS Binary Tree Level Order Traversal II

最新推荐文章于 2024-02-12 01:19:39 发布

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本文详细介绍了使用BFS算法反转二叉树层序遍历的顺序，通过C++代码实现，提供了从根节点到叶子节点的反向遍历路径。

思路：

BFS。

上一题的基础上反转一下结果数组即可。

std:reverse()

Reverse range

Reverses the order of the elements in the range [first,last).

#include <algorithm>

template <class BidirectionalIterator>
  void reverse (BidirectionalIterator first, BidirectionalIterator last);

The function calls iter_swap to swap the elements to their new locations.

The behavior of this function template is equivalent to:

template <class BidirectionalIterator>
  void reverse (BidirectionalIterator first, BidirectionalIterator last)
{
  while ((first!=last)&&(first!=--last)) {
    std::iter_swap (first,last);
    ++first;
  }
}

/**
 * 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;
        queue<TreeNode*> current, nextLevel;
        vector<int> level;
        current.push(root);
        while(!current.empty()) {
            while(!current.empty()) {
                TreeNode *node = current.front();
                current.pop();
                level.push_back(node->val);
                if(node->left != NULL) nextLevel.push(node->left);
                if(node->right != NULL) nextLevel.push(node->right);
            }
            res.push_back(level);
            level.clear();
            swap(current, nextLevel);
        }
        reverse(res.begin(), res.end());
        return res;
    }
};