[GO] Math | O(logn)
Code\n\n/*\na => Permutaions of l=0  \nb => Permutaions of e=0, e=1\nc => Permutaions of t=0\n\nMath : \nf(n) = 26^n - f(a U b U c)\n\nVenn diagram:\nf(a U b U 
# Code\n```\n/*\na => Permutaions of l=0  \nb => Permutaions of e=0, e=1\nc => Permutaions of t=0\n\nMath : \nf(n) = 26^n - f(a U b U c)\n\nVenn diagram:\nf(a U b U c) = f(a) + f(b) + f(c) - f(a \u2229 b) - f(b \u2229 c) - f(a \u2229 c) + f(a \u2229 b \u2229 c) \n\nf(n) = 26^n - (25^n + 25^n + n*25^(n-1) + 25^n - 24^n - 24^n - n*24^(n-1) - 24^n - n*24^(n-1) + 23^n + n*23^n-1)\nf(n) = 26^n - (75+n)*25^n-1 + (72+2n)*24^n-1 - (23+n) 23^n-1\n*/ \nfunc stringCount(n int) int {\n    var pow func (a, b int) int\n    pow = func (a, b int) int {\n        if b == 0 {\n            return 1\n        }\n        ans := 1\n        if b % 2 == 1 {\n            ans = a\n        }\n        return (ans  * pow((a*a) % 1000000007, b/2)) % 1000000007\n    }\n    res := (pow(26, n) - (75+n)*pow(25, n-1) + (72+2*n)*pow(24, n-1) - (23+n)*pow(23, n-1)) % 1000000007\n    return (res + 1000000007) % 1000000007\n}\n```
['Math', 'Go']