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class Solution {
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  constructor(nums) {
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    this.map = new Map();
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    this.sum = 0;
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    nums.forEach((num, i) => {
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      this.sum += num;
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      this.map.set(this.sum, i);
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    });
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  }
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  pickIndex = function () {
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    const random_sum = Math.floor(Math.random() * this.sum);
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    return this.fetchIndexUsingBS(random_sum);
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  };
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  fetchIndexUsingBS = function (target) {
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    const sums = Array.from(this.map.keys());
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    let lo = 0,
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      hi = sums.length - 1,
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      mid;
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    while (lo <= hi) {
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      mid = Math.floor((hi - lo) / 2) + lo;
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      // if((mid === 0 || sums[mid - 1] < target) && sums[mid] >= target) {
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      //     return this.map.get(sums[mid]);
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      // }
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      if (sums[mid] > target) {
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        hi = mid - 1;
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      } else {
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        lo = mid + 1;
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      }
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    }
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    return lo;
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  };
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}