Binary Tree Level Order Traversal

描述

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:Given binary tree {3,9,20,#,#,15,7},

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

return its level order traversal as:

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

分析

无

递归版

  1. // Binary Tree 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<vector<int> > levelOrder(TreeNode *root) {
  6.         vector<vector<int>> result;
  7.         traverse(root, 1, result);
  8.         return result;
  9.     }
  11.     void traverse(TreeNode *root, size_t level, vector<vector<int>> &result) {
  12.         if (!root) return;
  14.         if (level > result.size())
  15.             result.push_back(vector<int>());
  17.         result[level-1].push_back(root->val);
  18.         traverse(root->left, level+1, result);
  19.         traverse(root->right, level+1, result);
  20.     }
  21. };

迭代版

  1. // Binary Tree Level Order Traversal
  2. // 迭代版，时间复杂度O(n)，空间复杂度O(1)
  3. class Solution {
  4. public:
  5.     vector<vector<int> > levelOrder(TreeNode *root) {
  6.         vector<vector<int> > result;
  7.         queue<TreeNode*> current, next;
  9.         if(root == nullptr) {
  10.             return result;
  11.         } else {
  12.             current.push(root);
  13.         }
  15.         while (!current.empty()) {
  16.             vector<int> level; // elments in one level
  17.             while (!current.empty()) {
  18.                 TreeNode* node = current.front();
  19.                 current.pop();
  20.                 level.push_back(node->val);
  21.                 if (node->left != nullptr) next.push(node->left);
  22.                 if (node->right != nullptr) next.push(node->right);
  23.             }
  24.             result.push_back(level);
  25.             swap(next, current);
  26.         }
  27.         return result;
  28.     }
  29. };

相关题目

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