算法练习之双指针算法

前言

一、移动零【做题链接】

二、复写零【做题链接】

三、快乐数【做题链接】

四、盛水最多的容器【做题链接】

五、查找总价值为目标值的两件商品【做题链接】

六、三数之和【做题链接】

七、四数之和【做题链接】

八、有效三角形的个数【做题链接】

总结

前言

欢迎感兴趣的小伙伴交流学习。

一、移动零【做题链接】

题目代码：

class Solution {
public:
    void moveZeroes(vector<int>& nums) 
    {
        int green_pointer = 0;
        int blue_pointer = -1;

        for (; green_pointer < nums.size(); green_pointer++)
        {
            if (nums[green_pointer] != 0)
            {
                blue_pointer++;
                
                int tmp;
                tmp = nums[green_pointer];
                nums[green_pointer] = nums[blue_pointer];
                nums[blue_pointer] = tmp;
            }
        }
    }
};

二、复写零【做题链接】

题目代码：

class Solution {
public:
    void duplicateZeros(vector<int>& arr)
    {
        int write = arr.size() - 1;
        int count_write_zore = 0;
        int read;
        int i = 0;
        for (; count_write_zore < arr.size(); i++)
        {
            count_write_zore++;
            if (arr[i] == 0)
            {
                count_write_zore++;
            }
        }
        read = i - 1;
      
        for (; read >= 0; read--)
        {
            arr[write--] = arr[read];
            if (arr[read] == 0 && (read != i - 1 || count_write_zore == arr.size()))
            {
                arr[write--] = arr[read];
            }
        }
    }
};

三、快乐数【做题链接】

题目代码：

class Solution {
public:
    int count(int n)
    {
        int ret=0;
        while(n%10!=0||n/10)
        {
            ret+=pow(n%10,2);
            n/=10;
        }
        cout<<ret<<" ";
        return ret;
    }

    bool isHappy(int n) 
    {
        int fast_pointer=count(count(n));
        int slow_pointer=n;

        while(fast_pointer!=slow_pointer)
        {   

            fast_pointer=count(count(fast_pointer));
            cout<<"(";
            slow_pointer=count(slow_pointer);
            cout<<")";
        }
       
        if(fast_pointer==1)
        {
            return true;
        }
        return false;
    }
};

四、盛水最多的容器【做题链接】

class Solution {
public:
    int maxArea(vector<int>& height) 
    {
        int left=0;
        int right=height.size()-1;
        int ret=0;

        while(left<right)
        {
           ret=max(ret,(right-left)*min(height[right],height[left]));
           if(height[left]<height[right])
           {
            left++;
           }
           else if(height[left]>=height[right])
           {
            right--;
           }
           cout<< ret<<" "<<left<<" ";
        }
        return ret;
    }
};

五、查找总价值为目标值的两件商品【做题链接】

class Solution 
{
public:
    vector<int> twoSum(vector<int>& price, int target) 
    {
        int left=0;
        int right=0;
        vector<int> ret;
        for(int i=0;i<price.size();i++)
        {
            left=i;
            right=price.size()-1;
            int need=target-price[i];
            while(left<right)
            {
                if(need>price[right])
                {
                    break;
                }
                else if(need<price[left])
                {
                    break;
                }
                else
                {
                    if(need==price[left]||need==price[right])
                    {
                        ret.push_back(price[i]);
                        if(need==price[left])
                        {
                            ret.push_back(price[left]);
                        }
                        else
                        {
                            ret.push_back(price[right]);
                        }
                        return ret;
                    }
                    left++;
                    right--;
                }
            }
        }
        return ret;
    }
};

六、三数之和【做题链接】

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums)
    {
        vector<vector<int>> ret;
        sort(nums.begin(), nums.end());    
        for (int i = 0; i < nums.size(); i++)
        {
            int left = i + 1;
            int right = nums.size() - 1;
            int target = -nums[i];
            if (i!=0&&nums[i] == nums[i - 1])
            {
                continue;
            }
            while (left < right)
            {
                
                if (target == nums[left]+ nums[right])
                {
                    if ((left!=i+1)&&nums[left]==nums[left-1]||(right!=nums.size()-1)&&nums[right]==nums[right+1])
                    {
                        if (nums[left] == nums[left - 1])
                        {
                            left++;
                        }
                        if (nums[right] == nums[right + 1])
                        {
                            right--;
                        }
                        continue;
                    }
                    ret.push_back({ nums[i],nums[left],nums[right]});
                    left++;
                    right--;
                }
                else if (target > nums[left] + nums[right])
                {
                    left++;
                }
                else if (target < nums[left]+ nums[right])
                {
                    right--;
                }
            }
        }
        return ret;
    }
};

七、四数之和【做题链接】

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) 
    {
        vector<vector<int>> ret;
        sort(nums.begin(),nums.end());
        for(int i=0;i<nums.size();i++)
        {
            long long tmp1=target-nums[i];
            if(i!=0&&nums[i]==nums[i-1])
            {
                continue;
            }
            for(int j=i+1;j<nums.size();j++)
            {
                long long tmp2=tmp1-nums[j];
                if(j!=i+1&&nums[j]==nums[j-1])
                {
                    continue;
                }

                int left=j+1;
                int right=nums.size()-1;
                while(left<right)
                {
                    if(tmp2==nums[left]+nums[right])
                    {
                        if((left!=j+1&&nums[left]==nums[left-1])||(right!=nums.size()-1&&nums[right]==nums[right+1]))
                        {
                            if(nums[left]==nums[left-1])left++;
                            if(nums[right]==nums[right+1])right--;
                            continue;
                        }
                        ret.push_back({nums[i],nums[j],nums[left],nums[right]});
                        left++;
                        right--;
                    }
                    else if(tmp2>nums[left]+nums[right])
                    {
                        left++;
                    }
                    else if(tmp2<nums[left]+nums[right])
                    {
                        right--;
                    }
                }
            }
        }
        return ret;
    }
};

八、有效三角形的个数【做题链接】

class Solution {
public:
    int triangleNumber(vector<int>& nums) {
        if (nums.size() < 3) return 0;
        sort(nums.begin(), nums.end());
        int ret = 0;
        int n = nums.size();
        for (int i = n - 1; i >= 2; --i){//固定住最长的那条边
            int l = 0, r = i - 1;//l,r分别代表两条边
            while (l < r){
                if (nums[l] + nums[r] > nums[i]){
                    ret += r - l;
                    --r;
                }else{
                    ++l;
                }
            }
        }

        return ret;
    }
};