Binary Tree Preorder Traversal

https://leetcode.com/problems/binary-tree-preorder-traversal/description/

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

Thoughts

一个栈用来做backtracking。先是当前点加入到res, 然后是左节点, 再然后是右节点, 因此处理完当前节点后, 先把右节点压入, 再把左节点压入, 下次循环体就会先把左节点抛出并把左节点的子树压入, 直到左子树全部结束, 这时右子树开始依次往外蹦.

Code

/**
 * 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 LinkedList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node != null) {
                res.add(node.val);
                stack.push(node.right);
                stack.push(node.left);
            }
        }

        return res;
    }
}

Analysis

时间复杂度是O(n), 空间O(h).