leetcode-830-Positions-of-Large-Groups

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描述

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:

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

Example 2:

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

Example 3:

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

Note: 1 <= S.length <= 1000

分析

一前一后双指针，一次遍历即可。

解决方案1(Java)

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class Solution {
    public List<List<Integer>> largeGroupPositions(String S) {
        S = S + '#';
        List<List<Integer>> result = new LinkedList<>();
        for (int fastI = 0, slowI = 0; fastI < S.length(); fastI++) {
            if (S.charAt(fastI) != S.charAt(slowI)) {
                if ((fastI-slowI) >= 3) {
                    result.add(Arrays.asList(new Integer[]{slowI, fastI-1}));
                }
                slowI = fastI;
            }
        }
        return result;
    }
}

题目来源