题解 | #二叉树的遍历#

二叉树的前序、中序、后序遍历指的是对于根节点的顺序
采用递归的方式解决，两个判断左右非null的语句+读取当前节点的语句

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
     */
    List<Integer> list  = new ArrayList<Integer>();
    public int[][] threeOrders (TreeNode root) {
        // write code here
        preOlder(root);
        int len = list.size();
        int[][] nums = new int[3][len];
        int i=0;
        for(int n:list){
            nums[0][i] = n;
            i++;
        }
        list.clear();
        i=0;

        midOlder(root);
         for(int n:list){
            nums[1][i] = n;
            i++;
        }
        list.clear();
        i=0;

        postOlder(root);
        for(int n:list){
            nums[2][i] = n;
            i++;
        }
        list.clear();
        i=0;

        return nums;
    }
    public void preOlder(TreeNode root){
        list.add(root.val);
        if(root.left!=null)
            preOlder(root.left);
        if(root.right!=null)
            preOlder(root.right);
    }
    public void midOlder(TreeNode root){

        if(root.left!=null)
            midOlder(root.left);
        list.add(root.val);
        if(root.right!=null)
            midOlder(root.right);
    }
    public void postOlder(TreeNode root){

        if(root.left!=null)
            postOlder(root.left);
        if(root.right!=null)
            postOlder(root.right);
        list.add(root.val);
    }
}