caluclate the number of triangles

triangles.py

from bisect import bisect_left


def countTriangleTripleIndex(a):
    a.sort()
    n, count = len(a), 0
    for i in range(n-2):
        j = i+1
        k = bisect_left(a, a[i]+a[j])-1
        while k < n:
            if k > j and a[i] + a[j] <= a[k]:
                count += k-j-1
                j += 1
            else:
                k += 1
        count += (k-j) * (k-j-1)/2
    return count

a = [2, 3, 4, 5]
print countTriangleTripleIndex(a)