[LeetCode] 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]
]

使用queue层次遍历二叉树，将每一层的元素放入一个数组中，然后将每一层的数组放入结果二维数组中，最后将结果二维数组反转即可。

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()) {
            int n = q.size();
            vector<int> cur;
            for (int i = 0; i != n; i++) {
                TreeNode* node = q.front();
                q.pop();
                cur.push_back(node->val);
                if (node->left != nullptr)
                    q.push(node->left);
                if (node->right != nullptr)
                    q.push(node->right);
            }
            res.push_back(cur);
        }
        reverse(res.begin(), res.end());
        return res;
    }
};
// 3 ms