## Saturday, May 26, 2018

### LeetCode 830 - Positions of Large Groups

https://leetcode.com/problems/positions-of-large-groups/description/
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]]```

public List<List<Integer>> largeGroupPositions(String S) {
List<List<Integer>> ans = new ArrayList();
int i = 0, N = S.length(); // i is the start of each group
for (int j = 0; j < N; ++j) {
if (j == N-1 || S.charAt(j) != S.charAt(j+1)) {
// Here, [i, j] represents a group.
if (j-i+1 >= 3)
i = j + 1;
}
}

return ans;
}

https://leetcode.com/problems/positions-of-large-groups/discuss/128957/C++JavaPython-Straight-Forward
For every groups, find its start index `i` and end index `j - 1`
Group length is `j - i`, if it's no less than 3, add (i, j) to result.
``````    public List<List<Integer>> largeGroupPositions(String S) {
int i = 0, j = 0, N = S.length();
List<List<Integer>> res = new ArrayList<>();
while (j < N) {
while (j < N && S.charAt(j) == S.charAt(i)) ++j;
if (j - i >= 3) res.add(Arrays.asList(i, j - 1));
i = j;
}
return res;
}
``````