双指针算法 + 位运算

1.双指针

                 

(1)题目示例1

                         

• 因为数据有唯一解且两个数组都为升序，指针 i 指向A数组开头，指针 j 指向 B 数组结尾 
• A[ i ] + B[ j ] > x : --j , A[ i ] + B[ j ] < x : ++i , A[ i ] + B[ j ] == x ,返回 i , j 

                        

#include<iostream>
#include<vector>

using namespace std;


int main()
{
    int n,m,x;
    cin >> n >> m >> x;
    vector<int> A(n,0);
    vector<int> B(m,0);
    
    for(int i = 0 ; i < n;++i)  cin >> A[i];
    for(int i = 0 ; i < m;++i)  cin >> B[i];
    
    int i = 0;
    int j = m-1;
    while(true)
    {
        int temp = A[i] + B[j];
        if(temp > x)
        {
            --j;
        }
        else if(temp < x)
        {
            ++i;
        }
        else
        {
            cout << i << " " << j;
            break;
        }
        
    }
    
    return 0;
}

(2)无重复字符的最长子串

                         

class Solution {
public:
    int lengthOfLongestSubstring(string s) 
    {
        int res = 0;
        int chars[128] = {0};  //记录某个字符的个数

        int j = 0;
        for(int i = 0 ; i < s.size() ;++i)
        {
           chars[s[i]]++;
           while(chars[s[i]] > 1) //如果该字符在 j ~ i 之间重复了，就要把最左边的该字符除去
           {
               chars[s[j]]--; //j不断右移除去 该字符
               ++j;
           }
           
           res = max(res,i-j+1); 
        }


        return res;

    }
};

(3)长度最小的子数组

                         

//1.最简版 -(self)
   int minSubArrayLen(int target, vector<int>& nums) 
    {
       int res = INT_MAX;
       
       int sum = 0;
       for(int i = 0 ,j = 0 ; i < nums.size() ;++i )
       {
           sum += nums[i];
           while(sum >= target)
           {
               res = min(res,i-j+1);
               sum -= nums[j];
               ++j;
           }
       }

       return res == INT_MAX ? 0 : res;
    }


//2.清晰版
      int minSubArrayLen(int target, vector<int>& nums) 
      {
        int left = 0, right = 0, res = INT_MAX, len = 0;
        int windows_sum = 0;

        for(int i= 0 ; i < nums.size() ;++i)
        {
            if(nums[i] >= target)
            return 1;
        }

        while(right < nums.size())//窗口区间为[left, right)
        {
            while(right < nums.size() && windows_sum < target) //增大窗口 ->右边先走
            {
                windows_sum += nums[right];//更新窗口状态
                right++;
            }

            while(windows_sum >= target)//收缩窗口 
            {
                len = right - left;
                res = min(res, len); //更新窗口大小
                windows_sum -= nums[left];//更新窗口状态
                left++;
            }
        }
        return res == INT_MAX? 0:res;
    }

        

(4)三数之和

                                 

• 先确定一个数，再利用双指针去找另外两个数

                 

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) 
    {
        //本题最重要的就是对a,b,c进行去重操作

        vector<vector<int>> res;
        int n = nums.size();
        sort(nums.begin(),nums.end()); //排序

        //小优化,最小的都 > 0
        if(nums[0] > 0)
        {
            return res;
        }

        //枚举a
        for(int i = 0 ; i < n ;++i)
        {
            //对a去重
           if(i && nums[i] == nums[i-1] ) continue;

           int left = i+1;
           int right = n-1;
           while(left < right)
           {
               if(nums[i] + nums[left] + nums[right] > 0)
               {
                   --right;
               }
               else if(nums[i] + nums[left] + nums[right] < 0)
               {
                   ++left;
               }
               else //找到一组结果
               {
                  //要对b,c去重
                  //去重逻辑应该放在找到一个三元组之后，对b 和 c去重
                  while(left < right && nums[left] == nums[left+1]) ++left;
                  while(left < right && nums[right] == nums[right-1]) --right;

                  res.push_back({nums[i], nums[left] ,nums[right]});

                  //找到答案时，双指针同时收缩
                  left++;
                  right--;
               }
           }
        }

        return res;
    }
};

        

                        

2.位运算

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    int n;
    cin >> n;
    
    vector<int> a(n,0);
    for(int i = 0 ; i < n ;++i) cin >> a[i];
    
    for(int i = 0 ; i < n ;++i)
    {
        int temp = a[i];
        int count = 0;
        while(temp)
        {
            if(temp & 1)
            {
                count++;
            }
            temp = temp >> 1;
        }
        cout << count << " ";
    }
    
    return 0;
}