Count Nice Pairs in an Array LeetCode solution in C++

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const MOD = 1e9 + 7;
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const countNicePairs = (a) => {
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  let m = new Map();
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  let res = 0;
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  for (const e of a) {
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    let target = e - rev(e);
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    let cnt = m.get(target) || 0;
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    res += cnt;
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    m.set(target, cnt + 1);
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  }
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  return res % MOD;
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};
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/* Another version
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const countNicePairs = (a) => {
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    let m = new Map();
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    let res = 0;
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    for (const e of a) {
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        let target = e - rev(e);
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        let cnt = m.get(target) + 1 || 1;
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        m.set(target, cnt);
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        res += cnt - 1;
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    }
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    return res % MOD;
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};
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*/
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const rev = (num) => {
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  let s = num + '';
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  let res = '';
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  let n = s.length;
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  for (let i = n - 1; ~i; i--) res += s[i];
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  return Number(res);
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};