144.二叉树的前序遍历.java

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;
/*
 * @lc app=leetcode.cn id=144 lang=java
 *
 * [144] 二叉树的前序遍历
 */

import javax.swing.tree.TreeNode;

// @lc code=start
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if (root == null) return ans;

        Deque<TreeNode> stack = new LinkedList<>();
        stack.push(root);

        while (!stack.isEmpty()) {
            TreeNode tmp = stack.pop();
            if (tmp != null) {
                if (tmp.right != null) {
                    stack.push(tmp.right);
                }
                if (tmp.left != null) {
                    stack.push(tmp.left);
                }

                stack.push(tmp);
                stack.push(null);
            } else {
                ans.add(stack.pop().val);
            }
        }
        return ans;
    }

}
// @lc code=end