剑指offer-两个链表的第一个公共结点

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
import java.util.HashSet;
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        //记录下list1的所有节点
        HashSet<ListNode> set = new HashSet<>();
        ListNode cur = pHead1;
        while(cur != null){
            set.add(cur);
            cur = cur.next;
        }
        cur = pHead2;
        while(cur != null){
            if(set.contains(cur)){
                return cur;
            }
            cur = cur.next;
        }
        return null;
    }
}

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        //获取两个链表的长度
        int len1 = getLength(pHead1);
        int len2 = getLength(pHead2);
        //让长链表先走相差的距离
        if(len1 >= len2){
            ListNode cur = pHead1;
            int dis = len1 - len2;
            while(dis>0){
                pHead1=pHead1.next;
                dis--;
            }
        }else{
            int dis = len2 - len1;
            while(dis>0){
                pHead2=pHead2.next;
                dis--;
            }
        }
        //遍历到公共节点
        while(pHead1!=pHead2){
            pHead1 = pHead1.next;
            pHead2 = pHead2.next;
        }
        return pHead1;
    }
    
    public int getLength(ListNode cur){
        int len =0;
        while(cur != null){
            len++;
            cur = cur.next;
        }
        return len;
    }
}