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LeetCode算法笔记--环形链表

算法题 热题HOT100 快慢指针
字数统计: 279阅读时长: 1 min
2021/01/03 Share

LeetCode算法笔记-Day57

141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where the tail connects to. If pos == -1, then there is no cycle in the linked list.

Can you solve it using O(1) (i.e. constant) memory?

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example:
  
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

方法一：快慢指针

Analyse:

  • 类似“追及问题”
  • 两个人在环形跑道上赛跑，同一个起点出发，一个跑得快一个跑得慢，在某一时刻，跑得快的必定会追上跑得慢的，只要是跑道是环形的，不是环形就肯定追不上。  

Answer：

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/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    if(!head) return false;
    let p_slow = head;
    let p_fast = head;

    while(p_fast){
        if(!p_fast.next) return false;
        p_fast = p_fast.next.next;
        p_slow = p_slow.next;
        if(p_slow == p_fast) return true;
    }

    return false;
};
CATALOG
  1. 1. LeetCode算法笔记-Day57
    1. 1.1. 141. Linked List Cycle
    2. 1.2. 方法一：快慢指针
      1. 1.2.1. Analyse:
      2. 1.2.2. Answer：
Total : 137
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