LeetCode160.相交链表【最通俗易懂版双指针】

160. 相交链表 - 力扣（LeetCode）

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    /**
    设 
        相交节点为 O，尾节点为 C，len(A->O) = a，len(B->O) = b，len(O->C) = c

    则
        1. len(A->C) = a + c
        2. len(B->C) = b + c
        1式和2式作差得 |a-b| = |lenA - lenB|
    
    故 
        让长链表先走|a-b|步后，再以步长1遍历，A与B将在O相遇
     */
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lenA = 0, lenB = 0;
        ListNode pA = headA, pB = headB;
        
        while (pA != null) {
            lenA ++;
            pA = pA.next;
        }
        while (pB != null) {
            lenB ++;
            pB = pB.next;
        }

        if (lenB > lenA) {
            for (int i = 0; i < lenB - lenA; i ++) {
                headB = headB.next;
            }
        } else {
            for (int i = 0; i < lenA - lenB; i ++) {
                headA = headA.next;
            }
        }

        while (headA != null && headB != null) {
                if (headA == headB) {
                    return headA;
                }
                headA = headA.next;
                headB = headB.next;
        }
        return null;
    }
}