NC45 实现二叉树先序，中序和后序遍历

问题

分别按照二叉树先序，中序和后序打印所有的节点

示例
输入：{1,2,3}
返回值：[[1,2,3],[2,1,3],[2,3,1]]

思路

import java.util.*;
/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */
public class Solution {
    /**
     * 
     * @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<>();
        preOrder(root, list1);
        inOrder(root, list2);
        postOrder(root, list3);
        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;
    }
    // 先序遍历
    public void preOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }
        list.add(root.val);
        preOrder(root.left, list);
        preOrder(root.right, list);
    }
    // 中序遍历
    public void inOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }
        inOrder(root.left, list);
        list.add(root.val);
        inOrder(root.right, list);
    }
    // 后序遍历
    public void postOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }
        postOrder(root.left, list);
        postOrder(root.right, list);
        list.add(root.val);
    }
}