144 Binary Tree Preorder Traversal

144. Binary Tree Preorder Traversal

1. Question

Given a binary tree, return thepreordertraversal of its nodes' values.

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

   1
    \
     2
    /
   3

return[1,2,3].

2. Implementation

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();

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

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        TreeNode curNode = null;

        while (!stack.isEmpty()) {
            curNode = stack.pop();
            res.add(curNode.val);

            if (curNode.right != null) {
                stack.push(curNode.right);
            }

            if (curNode.left != null) {
                stack.push(curNode.left);
            }
        }
        return res;
    }
}

3. Time & Space Complexity

时间和空间复杂度都是O(n)