Binary Tree Level Order Traversal II

• Lintcode : 70 / Leetcode : 107
• Level : Medium

Problem

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).

Example
Example 1:

Input:
{1,2,3}
Output:
[[2,3],[1]]
Explanation:
    1
   / \
  2   3
it will be serialized {1,2,3}
level order traversal
Example 2:

Input:
{3,9,20,#,#,15,7}
Output:
[[15,7],[9,20],[3]]
Explanation:
    3
   / \
  9  20
    /  \
   15   7
it will be serialized {3,9,20,#,#,15,7}
level order traversal

Concept & Algorithm

筆記

Time Complexity & Space Complexity

time : O(n)
space: O(n)

Answer

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode * root) {
        vector<vector<int>> ans;
        if (root == nullptr) return ans;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()) {
            int s = q.size();
            vector<int> res;
            for (int i = 0; i < s; i++) {
                TreeNode* p = q.front(); q.pop();
                res.push_back(p -> val);
                if (p -> left) q.push(p -> left);
                if (p -> right) q.push(p -> right);
            }
            ans.insert(ans.begin(), res);
        }
        return ans;
    }
};

tags: BFS

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