830. Positions of Large Groups

Easy

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.

解題

一開始的醜陋程式碼：

func largeGroupPositions(s string) [][]int {
    start:=0
    end:=0
    ans := make([][]int, 0)
    
    for i:=1; i<len(s); i++ {
        if s[i] == s[i-1] {
            end = i 
        } else {
            if end-start >= 2 { 
                ans = append(ans, []int{start, end})
            }
            start = i
            end = i
        }
    }

    if end-start >= 2 { 
        ans = append(ans, []int{start, end})
    }
    
    return ans
}

後來發現 end = i-1 以及 如果把 s 後面再加上一個空格，就可以不需要最後的 if 。

改進過後的程式碼：

Runtime: 0 ms, faster than 100.00%

Memory Usage: 2.7 MB, less than 48.48%

func largeGroupPositions(s string) [][]int {
    s = s+ " "
    
    start:=0
    ans := make([][]int, 0)
    
    for i:=1; i<len(s); i++ {
        if s[i] != s[i-1] {
            if i-start >= 3 { 
                ans = append(ans, []int{start, i-1})
            }
            start = i
        }
    }
    
    return ans
}