Leetcode 107. Binary Tree Level Order Traversal II (Easy) (cpp)

Leetcode 107. Binary Tree Level Order Traversal II (Easy) (cpp)

Tag: Tree, Breadth-first Search

Difficulty: Easy

/*

107. Binary Tree Level Order Traversal II (Easy)

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.
* 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) return res;
queue<TreeNode*> q;
q.push(root);
while (!q.empty()) {
vector<int> lev;
int _size = q.size();
for (int i = 0; i < _size; i++) {
TreeNode *temp = q.front();
q.pop();
lev.push_back(temp -> val);
if (temp -> left) q.push(temp -> left);
if (temp -> right) q.push(temp -> right);
}
res.push_back(lev);
}
reverse(res.begin(), res.end());
return res;
}
};