leetcode 109 : Convert Sorted List to Binary Search Tree

Question

public class Solution {
public TreeNode sortedListToBST(ListNode head) {
    if (head == null)
        return null;
    if (head.next == null)
        return new TreeNode (head.val);
    ListNode slow = head;
    ListNode fast = head;
    ListNode pre = null;
    ListNode mid = null;
    while (fast.next != null && fast != null){
        pre = slow;
        fast = fast.next.next;
        slow = slow.next;
    }
    mid = slow;
    pre.next = null;
    TreeNode root = new TreeNode(slow.val);
    root.right = sortedListToBST(mid.next);
    root.left = sortedListToBST(head);

    return root;
}

}

This is my solution but it shows: java.lang.NullPointerException

when i change while (fast.next != null && fast != null){ to while (fast != null && fast.next != null){ the solution is accepted

i don't know the differences between them.

`while (fast.next != null && fast != null){` will cause **NullPointerException** when `fast` is null,that's the reason — flyingfox, Dec 19 '18 at 09:30
when you check two conditions with &&, the following rule applies: If the first check results in false, the second (or all other) condition(s) won't be checked anymore. — Chris, Dec 19 '18 at 09:31
@OleV.V. What I want to say is that I lost a chance to earn reputation,nothing else — flyingfox, Dec 19 '18 at 09:35

score 0 · Accepted Answer · answered Dec 19 '18 at 09:32

0

It is how && operator works... if there is a condition (cond1 && cond2), compiler will check if cond1 is true.. if it is false it wont check for second condition.. it will check for 2nd condition only if 1st one is true.. in your case.. if fast is null.. fast.next will give null pointer... so you should always check if fast is null before checking fast.next...

answered Dec 19 '18 at 09:32

karthik manchala

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The questions is closed,why do you still can add an answer to it? I can not do it now – flyingfox Dec 19 '18 at 09:34
@lucumt haha, i think i added it just before closing it... i cannot add an answer now as well – karthik manchala Dec 19 '18 at 09:39
I was wondering too. As of writing the question stands as closed 10 mins ago and answered 7 mins ago. – Ole V.V. Dec 19 '18 at 09:41
1

@OleV.V. Seems like consistency issues with database :P maybe we can add one more question on stackoverflow to discuss this :D – karthik manchala Dec 19 '18 at 09:49
Thank you! i neglect it – catworm Dec 19 '18 at 10:34

leetcode 109 : Convert Sorted List to Binary Search Tree

1 Answers1