LetCode 103. 二叉樹的鋸齒形層次遍歷

/**
 * 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>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (root == NULL)
            return res;
        // 這裏換成雙向隊列，方便對頭或尾進行操作
        deque<TreeNode*> dq;
        dq.push_back(root);
        bool direction = true;
        while(!dq.empty()){
            // 這層樹的節點個數
            int nums = dq.size();
            vector<int> v;
            for (int i = 0; i < nums; i++){
                // 設置了一個變量來控制從左向右還是從右向左
                if (direction){
                    TreeNode* tmp = dq.front();
                    dq.pop_front();
                    if (tmp->left != NULL)
                        dq.push_back(tmp->left);
                    if (tmp->right != NULL)
                        dq.push_back(tmp->right);
                    v.push_back(tmp->val);
                }else{  
                    TreeNode* tmp = dq.back();
                    dq.pop_back();
                    if (tmp->right != NULL)
                        dq.push_front(tmp->right);
                    if (tmp->left != NULL)
                        dq.push_front(tmp->left);
                    v.push_back(tmp->val);
                }
            }
            res.push_back(v);
            direction = !direction;
        }
        return res;
    }
};

static int x=[](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();