160. Intersection of Two Linked Lists

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

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.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

題意：

給定兩個鏈表，求兩個鏈表的交點（如果沒有交點返回null）

思路：

對於兩個相交的鏈表的來說，較長的鏈表 比 較短的鏈表 多出的部分（後對齊）肯定不是相交的部分，所以把 較長的鏈表的向後移動，直到長度相同，再重新遍歷，相同即相遇。

public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lenA = length(headA), lenB = length(headB);
        // move headA and headB to the same start point
 

        while (lenA > lenB) {
            headA = headA.next;
            lenA--;
        }
        while (lenA < lenB) {
            headB = headB.next;
            lenB--;
        }
        // find the intersection until end
        while (headA != headB) {
            headA = headA.next;
            headB = headB.next;
        }
        return headA;
    }
    private int length(ListNode node) {
        int length = 0;
        while (node != null) {
            node = node.next;
            length++;
        }
        return length;
    }
}

leetcode-160. Intersection of Two Linked Lists