LeetCode - Algorithms - 160. Intersection of Two Linked Lists

Problem

160. Intersection of Two Linked Lists

Notes

  • If the two linked lists have no intersection at all, return null.
  • The linked lists must retain their original structure after the function returns.
  • Your code should preferably run in O(n) time and use only O(1) memory.

Java

my solution of O(n) time and O(1) memory

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null)
            return null;
        ListNode nodeA = headA;
        int lenA = 0;
        while (nodeA != null) {
            lenA++;
            nodeA = nodeA.next;
        }
        int lenB = 0;
        ListNode nodeB = headB;
        while (nodeB != null) {
            lenB++;
            nodeB = nodeB.next;
        }
        nodeA = headA;
        nodeB = headB;
        if (lenA < lenB) {
            for (int i = 0; i < lenB - lenA; i++) {
                nodeB = nodeB.next;
            }
        }
        if (lenA > lenB) {
            for (int i = 0; i < lenA - lenB; i++) {
                nodeA = nodeA.next;
            }
        }
        ListNode node = null;
        if (nodeA == nodeB)
            node = nodeA;
        while (nodeA != nodeB) {
            nodeA = nodeA.next;
            nodeB = nodeB.next;
            node = nodeA;
        }
        return node;
    }
}

Submission Detail

  • 46 / 46 test cases passed.
  • Runtime: 1 ms, faster than 98.53% of Java online submissions for Intersection of Two Linked Lists.
  • Memory Usage: 41.7 MB, less than 10.90% of Java online submissions for Intersection of Two Linked Lists.

stack

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import java.util.Deque;
import java.util.LinkedList;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null)
            return null;
        Deque<ListNode> stkA = new LinkedList<ListNode>();
        ListNode node = headA;
        while (node != null) {
            stkA.push(node);
            node = node.next;
        }
        Deque<ListNode> stkB = new LinkedList<ListNode>();
        node = headB;
        while (node != null) {
            stkB.push(node);
            node = node.next;
        }
        while (!stkA.isEmpty() && !stkB.isEmpty()) {
            if (stkA.peek() == stkB.peek()) {
                node = stkA.pop();
                stkB.pop();
            } else
                break;
        }
        return node;
    }
}

Submission Detail

  • 46 / 46 test cases passed.
  • Runtime: 2 ms, faster than 38.55% of Java online submissions for Intersection of Two Linked Lists.
  • Memory Usage: 42.6 MB, less than 11.53% of Java online submissions for Intersection of Two Linked Lists.