leetcode-830-easy

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.
Example 3:

Input: s = "abcdddeeeeaabbbcd"
Output: [[3,5],[6,9],[12,14]]
Explanation: The large groups are "ddd", "eeee", and "bbb".
Constraints:

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

思路一：直接遍历，在遍历的过程中统计前后下标的距离是否大于等于 2

    public List<List<Integer>> largeGroupPositions(String s) {
        char[] chars = s.toCharArray();

        int begin = 0;
        int end = 0;
        List<List<Integer>> result = new ArrayList<>();
        for (int i = 1; i < chars.length; i++) {

            if (chars[i] == chars[i - 1]) {
                end = i;
            } else {
                List<Integer> t = new ArrayList<>();
                t.add(begin);
                t.add(end);

                if (end - begin >= 2) {
                    result.add(t);
                }
                begin = i;
                end = i;

            }
        }
        if (end - begin >= 2) {
            List<Integer> t = new ArrayList<>();
            t.add(begin);
            t.add(end);
            result.add(t);
        }

        return result;
    }

思路二：看了一下题解，发现在字符串后面追加一个字符，可以省去最后一个串的判断