103. Binary Tree Zigzag Level Order Traversal

• Difficulty: Medium

• Topics: Stack, Tree, Breadth-first Search

• Similar Questions:

  • Binary Tree Level Order Traversal

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]
]

Solutions:

/**
 * 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>> ret;
        if (root == NULL)   return ret;
        bool forward = true;

        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            int n = q.size();
            vector<int> level(n);
            for (int i = 0; i < n; ++i) {
                TreeNode* node = q.front(); q.pop();
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
                if (forward) {
                    level[i] = node->val;
                } else {
                    level[n - 1 - i] = node->val;
                }
            }
            ret.push_back(level);
            forward = !forward;
        }

        return ret;
    }
};