Binary Tree Level Order Traversal II

描述

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

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

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

return its bottom-up level order traversal as:

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

分析

在上一题 Binary Tree Level Order Traversal 的基础上，reverse()一下即可。

递归版

  1. // Binary Tree Level Order Traversal II
  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> > levelOrderBottom(TreeNode *root) {
  6.         vector<vector<int>> result;
  7.         traverse(root, 1, result);
  8.         std::reverse(result.begin(), result.end()); // 比上一题多此一行
  9.         return result;
  10.     }
  12.     void traverse(TreeNode *root, size_t level, vector<vector<int>> &result) {
  13.         if (!root) return;
  15.         if (level > result.size())
  16.             result.push_back(vector<int>());
  18.         result[level-1].push_back(root->val);
  19.         traverse(root->left, level+1, result);
  20.         traverse(root->right, level+1, result);
  21.     }
  22. };

迭代版

  1. // Binary Tree Level Order Traversal II
  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> > levelOrderBottom(TreeNode *root) {
  6.         vector<vector<int> > result;
  7.         if(root == nullptr) return result;
  9.         queue<TreeNode*> current, next;
  10.         vector<int> level; // elments in level level
  12.         current.push(root);
  13.         while (!current.empty()) {
  14.             while (!current.empty()) {
  15.                 TreeNode* node = current.front();
  16.                 current.pop();
  17.                 level.push_back(node->val);
  18.                 if (node->left != nullptr) next.push(node->left);
  19.                 if (node->right != nullptr) next.push(node->right);
  20.             }
  21.             result.push_back(level);
  22.             level.clear();
  23.             swap(next, current);
  24.         }
  25.         reverse(result.begin(), result.end()); // 比上一题多此一行
  26.         return result;
  27.     }
  28. };

相关题目

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