题解 | #实现二叉树先序，中序和后序遍历#

先序：根左右
中序：左根右
后序：左右根
按照这个顺序进行递归遍历
最后进行整合返回成数组，这里参看我的另一个题解

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
     */
    ArrayList<Integer> One = new ArrayList<Integer>();
    ArrayList<Integer> Two = new ArrayList<Integer>();
    ArrayList<Integer> Three = new ArrayList<Integer>();
    public int[][] threeOrders(TreeNode root) {
        // write code here
        first(root);
        Second(root);
        Third(root);
        int n=One.size();
        int[][] count=new int[3][n];
        Integer[] one=One.toArray(new Integer[n]);
        Integer[] two=Two.toArray(new Integer[n]);
        Integer[] three=Three.toArray(new Integer[n]);
        for (int i = 0; i < n; i++) {
            count[0][i]=one[i];
            count[1][i]=two[i];
            count[2][i]=three[i];
        }
        return count;

    }

    void first(TreeNode root) {
        if (root==null)
            return;
        One.add(root.val);
        first(root.left);
        first(root.right);
    }
    void Second(TreeNode root) {
        if (root==null)
            return;
        Second(root.left);
        Two.add(root.val);
        Second(root.right);
    }
    void Third(TreeNode root) {
        if (root==null)
            return;
        Third(root.left);
        Third(root.right);
        Three.add(root.val);
    }
}