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var findTargetSumWays = function (nums, S) {
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  let sums = new Map([[0, 1]]);
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  for (let num of nums) {
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    const next = new Map();
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    for (let [sum, amount] of sums) {
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      const plus = sum + num;
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      const minus = sum - num;
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      next.set(plus, next.has(plus) ? next.get(plus) + amount : amount);
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      next.set(minus, next.has(minus) ? next.get(minus) + amount : amount);
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    }
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    sums = next;
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  }
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  return sums.has(S) ? sums.get(S) : 0;
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};