末尾有注意点 

167. 两数之和 II - 输入有序数组 - 力扣（LeetCode）

双指针：left、right

class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        int right = numbers.size() - 1;
        int left = 0;

        while(left < right)
        {
            int sum = numbers[left] + numbers[right];
            if(sum == target)
                return {left+1, right +1};

            if(sum < target)
            {
                left++;
            }
            else if(sum > target)
            {
                right--;
            }
        }
        return {-1, -1};
    }
};

15. 三数之和 - 力扣（LeetCode）

转化成两数之和

class Solution {
public:
    vector<vector<int>> help(vector<int>& nums, int left, int right, int target, int value)
    {
        vector<vector<int>>answer;
        while(left < right)
        {
            int sum = nums[left] + nums[right];
            if(sum == target)
            {
                vector<int> result;
                result.push_back(value);
                result.push_back(nums[left]);
                result.push_back(nums[right]);
                answer.push_back(result);

                while(left < right && nums[left] == nums[left+1])
                {
                    left++;
                }
                left++;
                while(left < right && nums[right] == nums[right-1])
                {
                    right--;
                }
                right--;
            }
            else if(sum < target)
            {
                left++;
            }
            else{
                right--;
            }
        }
        return answer;
    }



    vector<vector<int>> threeSum(vector<int>& nums) {
        sort(nums.begin(), nums.end());//快排
        int len = nums.size();
        vector<vector<int>>answer;
        vector<vector<int>> result;
        for(int i = 0; i < len; i++)
        {
            if(i >0 && nums[i] == nums[i-1]) 
            {
                continue;
            }
            result = help(nums, i+1, len-1, -nums[i], nums[i]);
            answer.insert(answer.end(), result.begin(), result.end());
        }
        return answer;
    }
};

注意：

如果写成(nums[i] == nums[i-1] && i >0)会报错，因为先执行的nums[i-1],i-1如果= -1就会报错，i>0放前面会先执行；