1
var largeGroupPositions = function (S) {
2
  let j = 0,
3
    res = [];
4
  for (let i = 0; i < S.length; i++) {
5
    if (S[i] !== S[i + 1]) {
6
      if (i - j + 1 >= 3) res.push([j, i]);
7
      j = i + 1;
8
    }
9
  }
10
  return res;
11
};