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 void rootFront(TreeNode currentNode, StringBuffer sb) {
        if (currentNode == null) return;
        sb.append(currentNode.val + ",");
        rootFront(currentNode.left, sb);
        rootFront(currentNode.right, sb);
    }

    public void rootMiddle(TreeNode currentNode, StringBuffer sb) {
        if (currentNode == null) return;
        rootMiddle(currentNode.left, sb);
        sb.append(currentNode.val + ",");
        rootMiddle(currentNode.right, sb);
    }

    public void rootBack(TreeNode currentNode, StringBuffer sb) {
        if (currentNode == null) return;
        rootBack(currentNode.left, sb);
        rootBack(currentNode.right, sb);
        sb.append(currentNode.val + ",");
    }

    public int[][] threeOrders(TreeNode root) {
        // write code here
        if (root == null) {
            return new int[3][0];
        } else {
            int[][] result = new int[3][];
            StringBuffer fsb = new StringBuffer();
            StringBuffer msb = new StringBuffer();
            StringBuffer bsb = new StringBuffer();
            rootFront(root, fsb);
            rootMiddle(root, msb);
            rootBack(root, bsb);

            int[] fa = Arrays.stream(fsb.deleteCharAt(fsb.length() -
                                     1).toString().split(",")).mapToInt(value -> Integer.valueOf(value)).toArray();
            int[] ma = Arrays.stream(msb.deleteCharAt(msb.length() -
                                     1).toString().split(",")).mapToInt(value -> Integer.valueOf(value)).toArray();
            int[] ba = Arrays.stream(bsb.deleteCharAt(bsb.length() -
                                     1).toString().split(",")).mapToInt(value -> Integer.valueOf(value)).toArray();


            result[0] = fa;
            result[1] = ma;
            result[2] = ba;
            return result;
        }

    }
}

其实遍历二叉树的递归写法大家都知道，这次主要提醒一下复制同样逻辑代码后相关变量要修改清楚。边复制边改，不然最后一次性改可能改漏。