LeetCode 103. Binary Tree Zigzag Level Order Traversal [Medium] [C++] 解題筆記

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

想法:

    這題也是採用 BFS traverse，然後用一個額外的變數紀錄目前是奇數層還是偶數層，遇到奇數層就將結果翻轉後

在存入。

完整程式碼:

class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        if (root == nullptr) { return {}; }
        vector<vector<int>> res;
        queue<TreeNode*> q;
        q.push(root);
        int cnt = 0;
        while (!q.empty()) {
            int q_size = q.size();
            vector<int> level;
            for (int i = 0; i < q_size; i++) {
                auto cur = q.front();
                q.pop();
                level.emplace_back(cur->val);
                if (cur->left != nullptr) { q.push(cur->left); }
                if (cur->right != nullptr) { q.push(cur->right); }
            }
            if (cnt++ % 2) { reverse(level.begin(), level.end()); }
            res.emplace_back(level);
        }
        return res;
    }
};