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
     */
    public int[][] threeOrders (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> returnList = new ArrayList<>();
        returnList.add(new ArrayList<Integer>());
        returnList.add(new ArrayList<Integer>());
        returnList.add(new ArrayList<Integer>());
        
        firstOrder(root,returnList.get(0));
        middleOrder(root,returnList.get(1));
        lastOrder(root,returnList.get(2));
        
        int[][] returnMatrix = new int[3][];
        
        for(int i = 0; i < 3; i++){
            returnMatrix[i] = new int[returnList.get(i).size()];
            for(int j = 0 ; j < returnList.get(i).size(); j++){
                returnMatrix[i][j] = returnList.get(i).get(j);
            }
        }
        
        return returnMatrix;
    }
    
    public void firstOrder(TreeNode root,ArrayList<Integer> scanList){
        if(root != null){
            scanList.add(root.val);
            firstOrder(root.left,scanList);
            firstOrder(root.right,scanList);
        }
    }
    
    public void middleOrder(TreeNode root,ArrayList<Integer> scanList){
        if(root != null){
            middleOrder(root.left,scanList);
            scanList.add(root.val);
            middleOrder(root.right,scanList);
        }
    }
    
    public void lastOrder(TreeNode root,ArrayList<Integer> scanList){
        if(root != null){
            lastOrder(root.left,scanList);
            lastOrder(root.right,scanList);
            scanList.add(root.val);
        }
    }
}