103. Binary Tree Zigzag Level Order Traversal

https://leetcode.com/submissions/detail/354923567/

Problem

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]
]

Solution

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> values;
        if (root == nullptr) return values;
        stack<TreeNode*> cur, next;
        cur.push(root);
        auto *curptr = &cur, *nextptr = &next;
        bool flag = false;
        while (!curptr->empty()) {
            flag = !flag;
            values.emplace_back(vector<int>());
            values.back().reserve(curptr->size());
            if (flag) {
                while (!curptr->empty()) {
                    values.back().emplace_back(curptr->top()->val);
                    if (curptr->top()->left) nextptr->push(curptr->top()->left);
                    if (curptr->top()->right) nextptr->push(curptr->top()->right);
                    curptr->pop();
                }
            } else {
                while (!curptr->empty()) {
                    values.back().emplace_back(curptr->top()->val);
                    if (curptr->top()->right) nextptr->push(curptr->top()->right);
                    if (curptr->top()->left) nextptr->push(curptr->top()->left);
                    curptr->pop();
                }
            }
            auto *temp = curptr;
            curptr = nextptr;
            nextptr = temp;
        }
        return values;
    }
};

• #tree

• #levelorder