Positions of Large Groups

830. Positions of Large Groups

In a string s of lowercase letters, these letters form consecutive groups of the same character.

For example, a string like s = “abbxxxxzyy” has the groups “a”, “bb”, “xxxx”, “z”, and “yy”.

A group is identified by an interval [start, end], where start and end denote the start and end indices (inclusive) of the group. In the above example, “xxxx” has the interval [3,6].

A group is considered large if it has 3 or more characters.

Return the intervals of every large group sorted in increasing order by start index.

### Example 1:
Input: s = "abbxxxxzzy"
Output: [[3,6]]
Explanation: "xxxx" is the only large group with start index 3 and end index 6.

### Example 2:
Input: s = "abc"
Output: []
Explanation: We have groups "a", "b", and "c", none of which are large groups.

Constraints:
1 <= s.length <= 1000
s contains lowercase English letters only.

/**
 * @param {string} s
 * @return {number[][]}
 */
 var largeGroupPositions = function(s) {
    let cnt = 0;
    const out = [];
    for (let i = 0; i < s.length; i++) {
       if (cnt === 0) {
           out.push([i]);
       }
       cnt++;
       
       if (s[i] !== s[i+1]) {
           const last = out.pop();
           if (cnt >= 3) {
               last.push(i);
               out.push(last);
           }
           cnt = 0;
       }
    }   
    return out;
};