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 List<Integer> preorderTraversal(TreeNode root) {
  5.         ArrayList<Integer> result = new ArrayList<>();
  6.         Stack<TreeNode> s = new Stack<>();
  7.         if (root != null) s.push(root);
  9.         while (!s.isEmpty()) {
  10.             final TreeNode p = s.pop();
  11.             result.add(p.val);
  13.             if (p.right != null) s.push(p.right);
  14.             if (p.left != null) s.push(p.left);
  15.         }
  16.         return result;
  17.     }
  18. }

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 List preorderTraversal(TreeNode root) {
  5.         ArrayList result = new ArrayList<>();
  6.         TreeNode cur = root;
  7.         TreeNode prev = null;
  9.         while (cur != null) {
  10.             if (cur.left == null) {
  11.                 result.add(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 != null && node.right != cur)
  18.                     node = node.right;
  20.                 if (node.right == null) { /* 还没线索化，则建立线索 */
  21.                     result.add(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 = null;
  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. }

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