Binary Tree Preorder Traversal

描述

Given a binary tree, return the preorder traversal of its nodes' values.

For example:Given binary tree {1,#,2,3},

  1.  1
  2.   \
  3.    2
  4.   /
  5.  3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

分析

用栈或者Morris遍历。

栈

  1. // Binary Tree Preorder Traversal
  2. // 使用栈，时间复杂度O(n)，空间复杂度O(n)
  3. class Solution {
  4. public:
  5.     vector<int> preorderTraversal(TreeNode *root) {
  6.         vector<int> result;
  7.         stack<const TreeNode *> s;
  8.         if (root != nullptr) s.push(root);
  10.         while (!s.empty()) {
  11.             const TreeNode *p = s.top();
  12.             s.pop();
  13.             result.push_back(p->val);
  15.             if (p->right != nullptr) s.push(p->right);
  16.             if (p->left != nullptr) s.push(p->left);
  17.         }
  18.         return result;
  19.     }
  20. };

Morris先序遍历

  1. // Binary Tree Preorder Traversal
  2. // Morris先序遍历，时间复杂度O(n)，空间复杂度O(1)
  3. class Solution {
  4. public:
  5.     vector preorderTraversal(TreeNode *root) {
  6.         vector result;
  7.         TreeNode *cur = root, *prev = nullptr;
  9.         while (cur != nullptr) {
  10.             if (cur->left == nullptr) {
  11.                 result.push_back(cur->val);
  12.                 prev = cur; /* cur刚刚被访问过 */
  13.                 cur = cur->right;
  14.             } else {
  15.                 /* 查找前驱 */
  16.                 TreeNode *node = cur->left;
  17.                 while (node->right != nullptr && node->right != cur)
  18.                     node = node->right;
  20.                 if (node->right == nullptr) { /* 还没线索化，则建立线索 */
  21.                     result.push_back(cur->val); /* 仅这一行的位置与中序不同 */
  22.                     node->right = cur;
  23.                     prev = cur; /* cur刚刚被访问过 */
  24.                     cur = cur->left;
  25.                 } else {    /* 已经线索化，则删除线索  */
  26.                     node->right = nullptr;
  27.                     /* prev = cur; 不能有这句，cur已经被访问 */
  28.                     cur = cur->right;
  29.                 }
  30.             }
  31.         }
  32.         return result;
  33.     }
  34. };

相关题目

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