3Sum Smaller

Posted 2020-08-09 amazingzoe

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?

 

Analyse: this problem only consider the index, rather than the value. Thus, we don‘t need to avoid duplicate values. 

Runtime: 13ms

 1 class Solution {
 2 public:
 3     int threeSumSmaller(vector<int>& nums, int target) {
 4         if (nums.size() < 3) return 0;
 5         
 6         sort(nums.begin(), nums.end());
 7         int result = 0;
 8         for (int i = 0; i < nums.size() - 2; i++) {
 9             int k = nums.size() - 1;
10             for (int j = i + 1; j < k; ) {
11                 if (nums[i] + nums[j] + nums[k] < target) {
12                     result += (k - j);
13                     j++;
14                 }
15                 else k--;
16             }
17         }
18         return result;
19     }
20 };