LeetCode: Binary Tree Preorder Traversal 解题报告

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     // sol1:
12     public List<Integer> preorderTraversal1(TreeNode root) {
13         List<Integer> ret = new ArrayList<Integer>();
14         
15         rec(root, ret);
16         return ret;
17     }
18     
19     public void rec(TreeNode root, List<Integer> ret) {
20         if (root == null) {
21             return;
22         }
23         
24         ret.add(root.val);
25         rec(root.left, ret);
26         rec(root.right, ret);
27     }
28     
29     public List<Integer> preorderTraversal(TreeNode root) {
30         List<Integer> ret = new ArrayList<Integer>();
31         
32         if (root == null) {
33             return ret;
34         }        
35         
36         Stack<TreeNode> s = new Stack<TreeNode>();
37         s.push(root);
38         
39         while (!s.isEmpty()) {
40             TreeNode cur = s.pop();
41             ret.add(cur.val);
42             
43             if (cur.right != null) {
44                 s.push(cur.right);
45             }
46             
47             if (cur.left != null) {
48                 s.push(cur.left);
49             }
50         }
51         
52         return ret;
53     }
54 }