题目：
Given an array of n integers nums and a target, find the number of index triplets i, j, k with 0 <= i < j < k < n that satisfy the condition nums[i] + nums[j] + nums[k] < target.
For example, given nums = [-2, 0, 1, 3], and target = 2.
Return 2. Because there are two triplets which sums are less than 2:
[-2, 0, 1]
[-2, 0, 3]
Follow up:
Could you solve it in O(n2) runtime?
方法一：
class Solution {
public:
int threeSumSmaller(vector<int>& nums, int target) {
int res = ;
sort(nums.begin(), nums.end());
for (int i = ; i < int(nums.size() - ); ++i) {
int left = i + , right = nums.size() - , sum = target - nums[i];
for (int j = left; j <= right; ++j) {
for (int k = j + ; k <= right; ++k) {
if (nums[j] + nums[k] < sum) ++res;
}
}
}
return res;
}
};
方法二：
- 双指针
class Solution2 {
public:
int threeSumSmaller(vector<int>& nums, int target) {
if (nums.size() < ) return ;
int res = , n = nums.size();
sort(nums.begin(), nums.end());
for (int i = ; i < n - ; ++i) {
int left = i + , right = n - ;
while (left < right) {
if (nums[i] + nums[left] + nums[right] < target) {
res += right - left;
++left;
} else {
--right;
}
}
}
return res;
}
};