LargeGroupPositions.java

package by.andd3dfx.string;

import java.util.ArrayList;
import java.util.List;

/**
 * <pre>
 * <a href="https://leetcode.com/problems/positions-of-large-groups/description/">Task description</a>
 *
 * 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.
 * </pre>
 *
 * @see <a href="https://youtu.be/RFTK3bFpLcs">Video solution</a>
 */
public class LargeGroupPositions {

    public List<List<Integer>> find(String s) {
        if (s == null || s.length() < 3) {
            return List.of();
        }

        var result = new ArrayList<List<Integer>>();
        var chars = s.toCharArray();

        var start = 0;
        var pos = 1;
        while (pos < chars.length) {
            if (chars[pos] != chars[pos - 1]) {
                addNewIntervalIfPossible(result, start, pos);
                start = pos;
            }
            pos++;
        }
        addNewIntervalIfPossible(result, start, pos);

        return result;
    }

    private void addNewIntervalIfPossible(List<List<Integer>> result, int start, int pos) {
        if (pos - start >= 3) {
            result.add(List.of(start, pos - 1));
        }
    }
}