LeetCode - 3Sum Smaller

3SumSmaller.java

// 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?

public int threeSumSmaller(int[] nums, int target) {
    int res = 0;
    if(nums == null || nums.length < 3) return res;
    Arrays.sort(nums);
    
    for(int i = nums.length - 1; i >= 2; i--){
        int left = 0, right = i - 1;
        while(left < right){
            if(nums[left] + nums[right] + nums[i] >= target){
                right--;
            } else {
               res += right - left;
               left++;
            }
        }
    }
    
    return res;
}