LeetCode: Binary Tree Preorder Traversal 解题报告

Binary Tree Preorder Traversal

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

For example:
Given binary tree {1,#,2,3},
   1
    \
     2
    /
   3
return [1,2,3].

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

SOLUTION1&2:

递归及非递归解法：

 /**
  * Definition for binary tree
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     // sol1:
     public List<Integer> preorderTraversal1(TreeNode root) {
         List<Integer> ret = new ArrayList<Integer>();

         rec(root, ret);
         return ret;
     }

     public void rec(TreeNode root, List<Integer> ret) {
         if (root == null) {
             return;
         }

         ret.add(root.val);
         rec(root.left, ret);
         rec(root.right, ret);
     }

     public List<Integer> preorderTraversal(TreeNode root) {
         List<Integer> ret = new ArrayList<Integer>();

         if (root == null) {
             return ret;
         }        

         Stack<TreeNode> s = new Stack<TreeNode>();
         s.push(root);

         while (!s.isEmpty()) {
             TreeNode cur = s.pop();
             ret.add(cur.val);

             if (cur.right != null) {
                 s.push(cur.right);
             }

             if (cur.left != null) {
                 s.push(cur.left);
             }
         }

         return ret;
     }
 }

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/tree/PreorderTraversal.java