107. Binary Tree Level Order Traversal II

---

 

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],

    3
   / \
  9  20
    /  \
   15   7

 

return its bottom-up level order traversal as:

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

 

 

 

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
import queue
class Solution:
    def levelOrderBottom(self, root: Optional[TreeNode]) -> List[List[int]]:

        q = queue.Queue()
        if root:
            q.put(root)
        res = []
        while q.qsize():
            sz = q.qsize()
            cur = []
            for i in range(sz):
                top = q.get()
                cur.append(top.val)
                if top and top.left: q.put(top.left)
                if top and top.right:q.put(top.right)
            res.append(cur)
        return res[::-1]

 

 

 

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

 

 

public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        List<List<Integer>> wrapList = new LinkedList<List<Integer>>();
        
        if(root == null) return wrapList;
        
        queue.offer(root);
        while(!queue.isEmpty()){
            int levelNum = queue.size();
            List<Integer> subList = new LinkedList<Integer>();
            for(int i=0; i<levelNum; i++) {
                if(queue.peek().left != null) queue.offer(queue.peek().left);
                if(queue.peek().right != null) queue.offer(queue.peek().right);
                subList.add(queue.poll().val);
            }
            wrapList.add(0, subList);
        }
        return wrapList;
    }
}