题解 | #实现二叉树先序，中序和后序遍历#

/*
 * 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> list = new ArrayList<>();
    public int[][] threeOrders (TreeNode root) {
        // write code here
        xianxu(root);
        int len = list.size();
        int[][] res = new int[3][len];
        for(int i = 0;i < len;i++){
            res[0][i] = list.get(i);
        }
        list.clear();
        zhongxu(root);
        for(int i = 0;i < len;i++){
            res[1][i] = list.get(i);
        }
        list.clear();
        houxu(root);
        for(int i = 0;i < len;i++){
            res[2][i] = list.get(i);
        }
        list.clear();
        return res;
    }
    public void xianxu(TreeNode root){
        if(root == null) return;
        list.add(root.val);
        xianxu(root.left);
        xianxu(root.right);
    }
    public void zhongxu(TreeNode root){
        if(root == null) return;
        zhongxu(root.left);
        list.add(root.val);
        zhongxu(root.right);
    }
    public void houxu(TreeNode root){
        if(root == null) return;
        houxu(root.left);
        houxu(root.right);
        list.add(root.val);
    }
}