830. Positions of Large Groups@python

Posted 2021-01-09 Chim

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:

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

原题地址: Positions of Large Groups

题意: 将数组分组,标记每一组的首尾索引值,返回分组长度等于3的索引值

难度: Easy

思路:

(1)遍历数组并用count数组标记,注意首尾边界

(2)遍历count,找出长度大于等于3的分组

代码:

class Solution(object):
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        count = []
        i = 0
        j = 1
        
        while j <= len(S):
            if j == len(S):
                count.append([i, j-1])
                break
                
            if S[i] == S[j]:
                j += 1
            else:
                count.append([i, j-1])
                i = j
        
        res = [] 
        for c in count:
            if c[1] - c[0] + 1 >= 3:
                res.append(c)
        return res

时间复杂度: O(n)

空间复杂度: O(n)

在上面的基础上优化一下,一次变量即可

代码:

class Solution(object):
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        res = []
        i, j = 0, 1
        n = len(S)
        while j < n:
            if S[j] != S[j-1]:   # 找到不相等的值
                count = j - i
                if count >= 3:
                    res.append([i, j-1])
                i = j
            j += 1
            
        if j - i >= 3:         # 处理边界问题
            res.append([i, j-1])
            
        return res