LeetCode(144):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?

 

非递归法（用栈来还原递归过程）：

public class Solution {
static  List<Integer> res = new ArrayList<>();
static  Stack<TreeNode> stack = new Stack<>();
public static List<Integer> preorderTraversal_inStack(TreeNode root) {
if(root == null) return new ArrayList<Integer>();
stack.push(root);
while(!stack.isEmpty()){
TreeNode tr = stack.pop();
res.add(tr.val);
if(tr.right!=null){
stack.push(tr.right);
}
if(tr.left!=null){
stack.push(tr.left);
}
}
return res;
}

 

递归法：

public static List<Integer> preorderTraversal(TreeNode root) {
if(root==null) return new ArrayList<Integer>();
res.add(root.val);
while(root.right!=null||root.left!=null){
preorderTraversal(root.right);
preorderTraversal(root.left);
}
return res;
}