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.

解法：计算两条链表长度。先移动较长那根链表n步（n是两者长度差），然后两根链表同步向尾端移动并且判断是否相等。有相等就是交点，否则返回null

public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
    int sizeA = getSize(headA);
    int sizeB = getSize(headB);
    if (sizeA > sizeB) {
        for (int i = 0; i < (sizeA-sizeB); i++) {
            headA = headA.next;
        }
    } else if (sizeB > sizeA) {
        for (int i = 0; i < (sizeB-sizeA); i++) {
            headB = headB.next;
        }
    }

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

private int getSize(ListNode head) {
    int count = 0;
    while (head != null) {
        count++;
        head = head.next;
    }
    return count;
}