leetcode 160.相交链表

题目就不写了，直接写解题思路。

刚开始想到的是暴力解法，双层循环遍历两个链表，然后对比是否有相同节点，有的话就直接返回，结果翻车了

var getIntersectionNode = function(headA, headB) {
        var startB = headB
        if(!headA || !headB){
            return null
        }
        while(headA){
            console.log('A'+headA.val);
            while (headB) {
                console.log('B'+headB.val);
                if(headB.val == headA.val) {
                    return headB.val
                }
                headB = headB.next
            }
            if(headA.val == headB.val){
                return headA.val
            }
            headB = startB
            headA = headA.next
        }
        return null
    };

判断到4就结束了，好吧，行不通。

第二种方法：

var getIntersectionNode = function(headA, headB) {
        if (!headA || !headB) {
            return null;
        }
        let a = headA,
            b = headB;
        // 相交则返回当前节点，否则返回 null
        // 当 a、b 还没相交时：
        // 1. 如果 a 为 null，将 headB 赋值给 a；
        // 2. 如果 b 为 null，将 headA 赋值给 b；
        while (a !== b) {
            a = a ? a.next : headB;
            b = b ? b.next : headA;
            console.log('a:'+JSON.stringify(a))
            console.log('b:' + JSON.stringify(b));
        }
        return a;
    };

因为两个链表的长度不一样，但是两个链表分别加上对方的链表长度就一样了，如下：

这样的话，如果链表有相交部分，那么在遍历的时候一定会相等的，等到链表相等的时候就退出循环返回相等部分的值。可能不理解怎么就相等了，原因是链表next指针在向后移动的时候，前面没有指针指向的部分会被删除掉，这样就只比较剩下的部分，直到两个相等了退出循环。若是不等，也会输出null，不会造成死循环

执行步骤如下：

    a:{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}
    b:{"val":0,"next":{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}}
    a:{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}
    b:{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}
    a:{"val":4,"next":{"val":5,"next":null}}
    b:{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}
    a:{"val":5,"next":null}
    b:{"val":4,"next":{"val":5,"next":null}}
    a:null
    b:{"val":5,"next":null}
    a:{"val":5,"next":{"val":0,"next":{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}}}
    b:null
    a:{"val":0,"next":{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}}
    b:{"val":4,"next":{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}}
    a:{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}
    b:{"val":1,"next":{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}}
    a:{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}
    b:{"val":8,"next":{"val":4,"next":{"val":5,"next":null}}}

来源：https://www.cnblogs.com/lyt0207/p/12364016.html

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