实现二叉树先序，中序和后序遍历

描述

分别按照二叉树先序，中序和后序打印所有的节点。

思路

递归。遍历将节点先加入到ArrayList中，再转换成数组，最后返回。

前序遍历

    private ArrayList<Integer> pre(ArrayList<Integer> arrayList, TreeNode root) {
        if (root==null) {
            return arrayList;
        }
        arrayList.add(root.val);
        if (root.left!=null) {
            arrayList = pre(arrayList,root.left);
        }
        if (root.right!=null) {
            arrayList = pre(arrayList,root.right);
        }
        return arrayList;
    }

中序遍历

    private ArrayList<Integer> mid(ArrayList<Integer> arrayList, TreeNode root) {
        if (root==null) {
            return arrayList;
        }
        if (root.left!=null) {
            arrayList = mid(arrayList,root.left);
        }
        arrayList.add(root.val);
        if (root.right!=null) {
            arrayList = mid(arrayList,root.right);
        }
        return arrayList;
    }

后序遍历

    private ArrayList<Integer> aft(ArrayList<Integer> arrayList, TreeNode root) {
        if (root==null) {
            return arrayList;
        }
        if (root.left!=null) {
            arrayList = aft(arrayList,root.left);
        }
        if (root.right!=null) {
            arrayList = aft(arrayList,root.right);
        }
        arrayList.add(root.val);
        return arrayList;
    }

主函数

    public int[][] threeOrders(TreeNode root) {
        ArrayList<Integer> preArrayList = pre(new ArrayList<>(), root);
        ArrayList<Integer> midArrayList = mid(new ArrayList<>(), root);
        ArrayList<Integer> aftArrayList = aft(new ArrayList<>(), root);
        int[] preOrder = new int[preArrayList.size()];
        int[] midOrder = new int[midArrayList.size()];
        int[] aftOrder = new int[aftArrayList.size()];
        for (int i = 0; i < preOrder.length; i++) {
            preOrder[i] = preArrayList.get(i);
            midOrder[i] = midArrayList.get(i);
            aftOrder[i] = aftArrayList.get(i);
        }
        int[][] orders = new int[][]{preOrder,midOrder,aftOrder};
        return orders;
    }