t36_两个链表的第一个公共节点.java

/**
 * 输入两个链表，找出它们的第一个公共结点。
 */

/**
 * 特殊值测试：有一个链表为空
 * 特殊值测试：无公共链表
 */

public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
 		int len1 = getLength(pHead1);
        int len2 = getLength(pHead2);
        if (len1 == 0 || len2 == 0) {
            return null;
        }
        //计算链表差值
        int dif = len1 > len2 ? len1 - len2 : len2 - len1;
        //长的链表先走dif步
        if (len1 > len2) {
            for (int i = 0; i < dif; i++) {
                pHead1 = pHead1.next;
            }
        } else {
            for (int i = 0; i < dif; i++) {
                pHead2 = pHead2.next;
            }
        }
        
        //一起往前走
        while (pHead1 != null && pHead2 != null && pHead1 != pHead2) {
            pHead1 = pHead1.next;
            pHead2 = pHead2.next;
        }
        
        if (pHead1 == null || pHead2 == null) {
            return null;
        } else {
            return pHead1;
        }
        
    }
    
    //获取链表长度
    int getLength(ListNode pHead) {
        int len = 0;
        while (pHead != null) {
            len++;
            pHead = pHead.next;
        }
        return len;
    }
}