1
var appealSum = function (s) {
2
  let ans = 0,
3
    n = s.length;
4
  let lastIndex = Array(26).fill(-1);
5
  for (let i = 0; i < n; i++) {
6
    let charcode = s.charCodeAt(i) - 97;
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    let lastIdx = lastIndex[charcode];
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    ans += (n - i) * (i - lastIdx);
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    lastIndex[charcode] = i;
10
  }
11
  return ans;
12
};