Solution144.java

package algorithm.leetcode;

import java.util.LinkedList;
import java.util.List;

/**
 * @author: mayuan
 * @desc: 二叉树的前序遍历
 * @date: 2019/03/09
 */
public class Solution144 {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new LinkedList<>();
        if (null == root) {
            return ans;
        }

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

        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            ans.add(cur.val);

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

        return ans;
    }

    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }
}