Find the Kth Smallest Sum of a Matrix With Sorted Rows LeetCode solution in JavaScript

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/**
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 * @param {number[][]} mat
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 * @param {number} k
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 * @return {number}
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 */
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var kthSmallest = function (mat, k) {
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  var m = mat.length;
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  var n = m ? mat[0].length : 0;
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  if (!m || !n) return -1;
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  var sums = [0];
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  for (var idx = 0; idx < m; idx++) {
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    var newSums = [];
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    for (var sum of sums) {
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      for (var i = 0; i < n && i < k; i++) {
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        var newSum = sum + mat[idx][i];
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        if (newSums.length < k) {
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          newSums.push(newSum);
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        } else if (newSum < newSums[k - 1]) {
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          newSums.pop();
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          newSums.push(newSum);
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        } else {
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          break;
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        }
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        newSums.sort((a, b) => a - b);
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      }
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    }
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    sums = newSums;
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  }
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  return sums[k - 1];
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};