Test for Overlapping Lists I

Leetcode #160

EPI 8.4

Description:

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

Example:

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.

Idea:

1. 计算两个的长度
2. 长的先走k步，然后两个一起走，check有没有相同的node。
3. 如果有，return。如果结束了还没有，return NULL。

Code:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        int lenA=lenList(headA);
        int lenB=lenList(headB);
        if(lenA>lenB){
            headA=advanceKStep(headA, lenA-lenB);
        }
        else{
            headB=advanceKStep(headB, lenB-lenA);
        }
        for(int i=0; i<min(lenA, lenB); i++){
            if(headA==headB){
                return headA;
            }
            headA=headA->next;
            headB=headB->next;
        }
        return NULL;
    }
    ListNode *advanceKStep(ListNode*head, int k){
        for(int i=0; i<k; i++){
            head=head->next;
        }
        return head;
    }
    int lenList(ListNode *head){
        int len=0;
        while(head!=NULL){
            ++len;
            head=head->next;
        }
        return len;
    }
};