# Leetcode: Positions of Large Groups

Positions of Large Groups Similar Problems:

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”.

Call a group large if it has 3 or more characters. We would like the starting and ending positions of every large group.

The final answer should be in lexicographic order.

Example 1:

```Input: "abbxxxxzzy"
Output: [[3,6]]
Explanation: "xxxx" is the single large group with starting  3 and ending positions 6.
```

Example 2:

```Input: "abc"
Output: []
Explanation: We have "a","b" and "c" but no large group.
```

Example 3:

```Input: "abcdddeeeeaabbbcd"
Output: [[3,5],[6,9],[12,14]]
```

Note: 1 <= S.length <= 1000

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

• Solution: Special treatment for last group
```// Blog link: https://code.dennyzhang.com/positions-of-large-groups
// Basic Ideas: two pointers
//   groups: [start, end]
// Complexity: Time O(n), Space O(n)
func largeGroupPositions(S string) [][]int {
res := [][]int{}
start := 0
for end := 0; end<len(S); end++ {
// find a new group
if S[end] != S[start] {
if end-start>=3 {
res = append(res, []int{start, end-1})
}
start = end
}
// last group
if S[end] == S[start] && end == len(S)-1 {
if end-start>=2 {
res = append(res, []int{start, end})
}
}
}
return res
}
```

• Solution: padding for last group
```// Blog link: https://code.dennyzhang.com/positions-of-large-groups
// Basic Ideas: two pointers
//   groups: [start, end]
// Complexity: Time O(n), Space O(n)
func largeGroupPositions(S string) [][]int {
res := [][]int{}
start := 0
// Note: ";" won't happen in S.
for end, ch := range S+";" {
if ch != rune(S[start]) {
if end-start >=3 {
res = append(res, []int{start, end-1})
}
start = end
}
}
return res
}
```

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