剑指Offer:两个链表的第一个公共结点Java/Python

1.题目描述

输入两个链表，找出它们的第一个公共结点。

2.算法描述

1.得到两个链表的长度。
2.让更长的链表的头指针先走两者长度的差值步。（此时两个头指针后面的长度是一样的）
3.让两个头指针同时向后移动，如果出现两个头指针相同，则是第一个公共结点，直接返回；如果头指针移动完也没有遇到两个头指针相同，则返回null。

3.代码描述

3.1.Java代码

/*
public class ListNode {
    int val;
    ListNode next = null;
    ListNode(int val) {
        this.val = val;
    }
    */
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        int size1 = getSize(pHead1);
        int size2 = getSize(pHead2);
        if(size1>size2)
            pHead1 = walkStep(pHead1, size1-size2);
        else
            pHead2 = walkStep(pHead2, size2-size1);
        while(pHead1!=null){
            if(pHead1 == pHead2)
                return pHead1;
            pHead1 = pHead1.next;
            pHead2 = pHead2.next;
        }
        return null;
    }
    private int getSize(ListNode head){
        int size = 0;
        while(head!=null){
            size++;
            head = head.next;
        }
        return size;
    }
    private ListNode walkStep(ListNode head, int step){
        while(step!=0){
            step--;
            head = head.next;
        }
        return head;
    }
}

3.2.Python代码

# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def FindFirstCommonNode(self, pHead1, pHead2):
        size1 = self.get_size(pHead1)
        size2 = self.get_size(pHead2)
        if size1>size2:
            pHead1 = self.walk_step(pHead1, size1-size2)
        else:
            pHead2 = self.walk_step(pHead2, size2-size1)
        while pHead1:
            if pHead1 == pHead2:
                return pHead1
            pHead1 = pHead1.next
            pHead2 = pHead2.next
        return None
    def get_size(self, head):
        size = 0
        while head:
            head = head.next
            size += 1
        return size
    def walk_step(self, head, step):
        while step:
            head = head.next
            step -= 1
        return head