[C++知识库]算法模板 双指针法

双指针法：用两个指针一个循环来完成（有时候两个循环才能完成的）一件事情

反转链表
https://leetcode-cn.com/problems/reverse-linked-list/submissions/

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* pre=NULL,*cur=head;
        while(cur!=NULL)
        {
            ListNode* temp=cur->next;
            cur->next=pre;
            pre=cur;
            cur=temp;
        }
        return pre;
    }
};

反转字符串
https://leetcode-cn.com/problems/reverse-string/submissions/

class Solution {
public:
    void swap(char& a,char& b)
    {
        char temp=a;
        a=b;
        b=temp;
    }
    void reverseString(vector<char>& s) {
            if(s.size()==1) return;
            for(int i=0,j=s.size()-1;i<=(s.size()-2)/2;i++,j--)
            {
                swap(s[i],s[j]);
            }
    }
};

移除元素
https://leetcode-cn.com/problems/remove-element/submissions/

class Solution {
public:
    int removeElement(vector<int>& nums, int val) {
        int slow=0,fast=0,len=nums.size();
        
        while(true)
        {
            while( (fast<=len-1)&&nums[fast]==val ) fast++;
            if(fast>len-1) break;
            nums[slow]=nums[fast];
            slow+=1;
            fast+=1;
        }
        return slow; 
    }
};

注意C语言的逻辑运算符（&& ||）是从左到右运算的