[LeetCode] Binary Tree Level Order Traversal II

最新推荐文章于 2021-11-18 16:57:58 发布

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最新推荐文章于 2021-11-18 16:57:58 发布

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本文详细介绍了如何使用宽度优先搜索（BFS）算法进行二叉树的底向上层遍历，包括具体实现步骤和示例代码解析。

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,#,#,15,7},

return its bottom-up level order traversal as:

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

宽搜，保存每一层的结果，最后将结果逆序

/**
 * Definition for binary tree
 * 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) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<vector<int>> ret;
        vector<TreeNode *> currentLevel;
        vector<TreeNode *> nextLevel;
        if(root == NULL) return ret;
        currentLevel.push_back(root);
       
        while(currentLevel.size() > 0) {
            nextLevel.clear();
            vector<int> v;
            for(int i = 0; i < currentLevel.size(); i++) {
                v.push_back(currentLevel[i]->val);
                TreeNode *tmp = currentLevel[i];
                if(tmp->left != NULL) nextLevel.push_back(tmp->left);
                if(tmp->right != NULL) nextLevel.push_back(tmp->right);
            }
            ret.push_back(v);
            currentLevel.clear();
            currentLevel.assign(nextLevel.begin(),nextLevel.end());
        }
        reverse(ret.begin(),ret.end());
        return ret;
    }
};