[LeetCode系列] 雙單鏈表共同節點搜索問題

找到兩個單鏈表的共同節點.

舉例來說, 下面兩個鏈表A和B:

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

共同節點爲c1.

 

分析:

共同節點距離A,B的起點headA, headB的距離差爲定值, 等於它們的各自總長的差值, 我們只需要求出這個差值, 把兩個鏈表的頭移動到距離c1相等距離的起點處即可.

代碼:

/**
 * 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 deltaLength = getLengthOfList(headA) - getLengthOfList(headB);
        // A > B
        if (deltaLength > 0)
        {
            while (deltaLength-- > 0) headA = headA->next;
        }
        // A < B
        else if (deltaLength < 0)
        {
            while (deltaLength++ < 0) headB = headB->next;
        }
        // now A and B has the same distance to intersection node
        while (NULL != headA && NULL != headB)
        {
            if (headA == headB)
                return headA;
            headA = headA->next;
            headB = headB->next;
        }
        return NULL;
    }
    
    int getLengthOfList(ListNode *head)
    {
        int res = 0;
        while (NULL != head)
        {
            res++;
            head = head->next;
        }
        return res;
    }
};