java实现二叉树 (前中后序遍历) 递归

今天刷了一道算法题, 814 二叉树剪枝
题解用的后序遍历
回顾一下 二叉树的前中后遍历 递归

Class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;

	TreeNode (){
    }
	
	TreeNode (int val) {
		this.val = val;
	}

	TreeNode (int val, TreeNode left, TreeNode right) {
		this.val = val;
		this.right = right;
		this.left = left
	}
	
}

List ans = new ArrayList<>();
public List traverse(TreeNode root){
	if (root == null) return null;
	ans.add(root.val);
	root.left = traverse(root.left);
	root.right = traverse(root.right);
	return ans;
}

List ans = new ArrayList<>();
public List traverse(TreeNode root){
	if (root == null) return null;
	root.left = traverse(root.left);
	ans.add(root.val);
	root.right = traverse(root.right);
	return ans;
}

List ans = new ArrayList<>();
public List traverse(TreeNode root){
	if (root == null) return null;
	root.left = traverse(root.left);
	root.right = traverse(root.right);
	ans.add(root.val);
	return ans;
}