Binary Tree Zigzag Level Order Traversal

描述

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,#,#,15,7},

  1.     3
  2.    / \
  3.   9  20
  4.     /  \
  5.    15   7

return its zigzag level order traversal as:

  1. [
  2.   [3],
  3.   [20,9],
  4.   [15,7]
  5. ]

分析

广度优先遍历，用一个bool记录是从左到右还是从右到左，每一层结束就翻转一下。

递归版

  1. // Binary Tree Zigzag Level Order Traversal
  2. // 递归版，时间复杂度O(n)，空间复杂度O(n)
  3. class Solution {
  4. public:
  5.     vector > zigzagLevelOrder(TreeNode *root) {
  6.         vector> result;
  7.         traverse(root, 1, result, true);
  8.         return result;
  9.     }
  11.     void traverse(TreeNode *root, size_t level, vector> &result,
  12.             bool left_to_right) {
  13.         if (!root) return;
  15.         if (level > result.size())
  16.             result.push_back(vector());
  18.         if (left_to_right)
  19.             result[level-1].push_back(root->val);
  20.         else
  21.             result[level-1].insert(result[level-1].begin(), root->val);
  23.         traverse(root->left, level+1, result, !left_to_right);
  24.         traverse(root->right, level+1, result, !left_to_right);
  25.     }
  26. };

迭代版

  1. // Binary Tree Zigzag Level Order Traversal
  2. // 广度优先遍历，用一个bool记录是从左到右还是从右到左，每一层结束就翻转一下。
  3. // 迭代版，时间复杂度O(n)，空间复杂度O(n)
  4. class Solution {
  5. public:
  6.     vector > zigzagLevelOrder(TreeNode *root) {
  7.         vector > result;
  8.         queue current, next;
  9.         bool left_to_right = true;
  11.         if(root == nullptr) {
  12.             return result;
  13.         } else {
  14.             current.push(root);
  15.         }
  17.         while (!current.empty()) {
  18.             vector level; // elments in one level
  19.             while (!current.empty()) {
  20.                 TreeNode* node = current.front();
  21.                 current.pop();
  22.                 level.push_back(node->val);
  23.                 if (node->left != nullptr) next.push(node->left);
  24.                 if (node->right != nullptr) next.push(node->right);
  25.             }
  26.             if (!left_to_right) reverse(level.begin(), level.end());
  27.             result.push_back(level);
  28.             left_to_right = !left_to_right;
  29.             swap(next, current);
  30.         }
  31.         return result;
  32.     }
  33. };

相关题目

原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/cpp/binary-tree/traversal/binary-tree-zigzag-level-order-traversal.html