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robherley/samAndSubstrings.js

Created March 16, 2018 18:29
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samAndSubstrings.js
const MOD = 1000000007;
const samSubstr = balls =>
[...balls].reduceRight(
(prev, digit, i) => {
return [
(prev[0] * 10 + 1) % MOD,
(prev[1] + ~~digit * prev[0] * (i + 1)) % MOD
];
},
[1, 0]
)[1];
console.log(samSubstr('16')); // 23
console.log(samSubstr('23')); // 164
console.log(samSubstr('1337')); // 1904
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