Eliminate Maximum Number of Monsters LeetCode solution in JavaScript

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/**
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 * @param {number[]} dist
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 * @param {number[]} speed
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 * @return {number}
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 */
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var eliminateMaximum = function (dist, speed) {
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  let res = 0;
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  let len = dist.length;
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  let map = new Map();
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  for (let i = 0; i < len; i++) {
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    // the last time to eliminate
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    let a = Math.ceil(dist[i] / speed[i]);
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    if (map.has(a)) {
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      let c = map.get(a);
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      c++;
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      map.set(a, c);
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    } else {
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      map.set(a, 1);
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    }
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  }
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  let keys = Array.from(map.keys());
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  keys.sort((a, b) => a - b);
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  // time to eliminate
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  let t = 0;
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  for (let i = 0; i < keys.length; i++) {
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    let c = map.get(keys[i]);
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    if (c > keys[i] - t) {
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      res += keys[i] - t;
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      break;
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    } else {
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      res += c;
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      t += c;
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    }
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  }
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  return res;
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};