java - 从树中返回值的字符串

Question

好吧，我想用中序、前序和后序遍历一棵树。我得到了遍历树的正确算法，但我需要以 "Node1 Node2 Node3" 的形式返回字符串。但我有"Node1Node2Node3" 。这是我的预订代码:

public String PreOrder() {
        /* TODO: Implement */
        StringBuilder string = new StringBuilder();
        preorderRecursive(string, root);
        return string.toString();
    }
    public StringBuilder preorderRecursive(StringBuilder string, Node current){
        if (current != null) {
        string.append(current.key);
        preorderRecursive(string, current.left);
        preorderRecursive(string, current.right);
        }
        return string;
    }

我收到错误org.junit.ComparisonFailure: expected:<3[ 1 2 8 4 5 7 9 ]10> but was:<3[1284579]10>

Answer 1

只需在 stringBuilder 中添加一个空格即可，

if (current != null) {
   string.append(current.key).append(" ");
   ...