Triangle

An array A consisting of N integers is given. A triplet (P, Q, R) is triangular if 0 ≤ P < Q < R < N and:

        A[P] + A[Q] > A[R],
        A[Q] + A[R] > A[P],
        A[R] + A[P] > A[Q].

For example, consider array A such that:
  A[0] = 10    A[1] = 2    A[2] = 5
  A[3] = 1     A[4] = 8    A[5] = 20

Triplet (0, 2, 4) is triangular.

Write a function:

    int solution(vector<int> &A);

that, given an array A consisting of N integers, returns 1 if there exists a triangular triplet for this array and returns 0 otherwise.

For example, given array A such that:
  A[0] = 10    A[1] = 2    A[2] = 5
  A[3] = 1     A[4] = 8    A[5] = 20

the function should return 1, as explained above. Given array A such that:
  A[0] = 10    A[1] = 50    A[2] = 5
  A[3] = 1

the function should return 0.

Write an efficient algorithm for the following assumptions:

        N is an integer within the range [0..100,000];
        each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].

#include <algorithm>

bool is_triangle(int64_t a, int64_t b, int64_t c)
{
    return (a + b > c) &&
           (b + c > a) &&
           (c + a > b);
}

int solution(vector<int> &A) {
    // write your code in C++11 (g++ 4.8.2)
    if (A.size() < 3) return 0;
    
    sort(A.begin(), A.end());
    
    int res = 0;
    for (int i = 0; i < int(A.size() - 2); i++) {
        if (is_triangle(A[i], A[i + 1], A[i + 2])) return 1;    
    }
    
    return 0;
}