题解 | #二叉树的前序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型一维数组
     */
    public int[] preorderTraversal (TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        preOrder3(root, list);
        return list.stream().mapToInt(Integer::intValue).toArray();
    }
    public void preOrder3(TreeNode root, List<Integer> list){
        Stack<TreeNode> stack = new Stack<>();
        if(root == null)
            return;

        stack.push(root);
        while(!stack.empty()){
            root = stack.pop();
            list.add(root.val);

            if(root.right != null)
                stack.push(root.right);
            if(root.left != null)
                stack.push(root.left);
        }
    }
    public void preOrder2(TreeNode root, List<Integer> list) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;

        while (!stack.empty() || root != null) {
            while (root != null && pre != root) {
                stack.push(root);
                list.add(root.val);
                pre = root;
                root = root.left;
            }
            while (true) {
                if (root != null && pre != root.right) {
                    root = root.right;
                    break;
                }
                pre = root;
                if (stack.empty())
                    break;
                root = stack.pop();
            }
        }
    }

    private void preOrder(TreeNode root, List<Integer> list) {
        if (root == null)
            return;

        list.add(root.val);
        preOrder(root.left, list);
        preOrder(root.right, list);
    }
}