题解 | #实现二叉树先序，中序和后序遍历#

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 the root of binary tree
     * @return int整型二维数组
     */
    public int[][] threeOrders (TreeNode root) {
        // write code here
        List<Integer> list1 = new ArrayList<>();
        List<Integer> list2 = new ArrayList<>();
        List<Integer> list3 = new ArrayList<>();
        //调用函数计算遍历结果
        preOrder(root, list1);
        inOrder(root, list2);
        postOrder(root, list3);
        //存放结果集
        int[][] res = new int[3][list1.size()];
        for (int i = 0; i < list1.size(); i++) {
            res[0][i] = list1.get(i);
            res[1][i] = list2.get(i);
            res[2][i] = list3.get(i);
        }
        return res;
    }
    private void preOrder(TreeNode root, List<Integer> list) {
        if(root==null) return;
        ArrayDeque<TreeNode> stack = new ArrayDeque<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode curr = stack.pop();
            list.add(curr.val);
            if (curr.right != null) {
                stack.push(curr.right);
            }
            if (curr.left != null) {
                stack.push(curr.left);
            }
        }
    }
    private void inOrder(TreeNode root, List<Integer> list) {
        if(root==null) return;
        ArrayDeque<TreeNode> stack = new ArrayDeque<>();
        TreeNode curr = root;
        while (!stack.isEmpty() || curr != null) {
            if (curr != null) {
                stack.push(curr);
                curr = curr.left;
            } else {
                curr = stack.pop();
                list.add(curr.val);
                curr = curr.right;
            }
        }
    }
    private void postOrder(TreeNode root, List<Integer> list) {
        if(root==null) return;
        ArrayDeque<TreeNode> stack = new ArrayDeque<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode curr = stack.pop();
            list.add(0,curr.val);
            if (curr.left != null) {
                stack.push(curr.left);
            }
            if (curr.right != null) {
                stack.push(curr.right);
            }
        }
    }
}