259. 3Sum Smaller

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`.

Example:

``````Input: _nums_ = [-2,0,1,3], and _target_ = 2
Output: 2
Explanation: 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 ( n 2) runtime?

``````// O(n^3)
class Solution {
public:
int threeSumSmaller(vector<int>& nums, int target) {
int res = 0;
sort(nums.begin(), nums.end());
for (int i = 0; i < int(nums.size() - 2); ++i) {
int left = i + 1, right = nums.size() - 1, sum = target - nums[i];
for (int j = left; j <= right; ++j) {
for (int k = j + 1; k <= right; ++k) {
if (nums[j] + nums[k] < sum) ++res;
}
}
}
return res;
}
};
``````

``````// O(n^2)
class Solution {
public:
int threeSumSmaller(vector<int>& nums, int target) {
if (nums.size() < 3) return 0;
int res = 0, n = nums.size();
sort(nums.begin(), nums.end());
for (int i = 0; i < n - 2; ++i) {
int left = i + 1, right = n - 1;
while (left < right) {
if (nums[i] + nums[left] + nums[right] < target) {
res += right - left;
++left;
} else {
--right;
}
}
}
return res;
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/259

3Sum Closest

3Sum

Two Sum Less Than K

https://leetcode.com/problems/3sum-smaller/

https://leetcode.com/problems/3sum-smaller/discuss/68817/Simple-and-easy-understanding-O(n2)-JAVA-solution

https://leetcode.com/problems/3sum-smaller/discuss/68820/Accepted-and-Simple-Java-O(n2)-solution-with-detailed-explanation

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