3Sum With Multiplicity LeetCode solution in JavaScript

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/**
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 * @param {number[]} arr
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 * @param {number} target
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 * @return {number}
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 */
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var threeSumMulti = function (arr, target) {
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  let count = 0;
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  arr.sort((a, b) => a - b);
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  for (let i = 0; i < arr.length - 2; i++) {
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    let j = i + 1,
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      k = arr.length - 1;
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    while (j < k) {
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      let sum = arr[i] + arr[j] + arr[k];
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      if (sum < target) {
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        j++;
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      } else if (sum > target) {
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        k--;
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      } else {
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        if (arr[j] !== arr[k]) {
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          let j1 = j,
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            k1 = k;
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          while (arr[j] === arr[j1]) {
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            j1++;
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          }
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          while (arr[k] === arr[k1]) {
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            k1--;
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          }
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          count += (j1 - j) * (k - k1);
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          j = j1;
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          k = k1;
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        } else {
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          for (let n = 1; n <= k - j; n++) {
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            count += n;
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          }
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          break;
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        }
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      }
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    }
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  }
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  return count % (10 ** 9 + 7);
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};