[leetcode] 984. String Without AAA or BBB

Description

Given two integers a and b, return any string s such that:

• s has length a + b and contains exactly a ‘a’ letters, and exactly b ‘b’ letters,
• The substring ‘aaa’ does not occur in s, and
• The substring ‘bbb’ does not occur in s.

Example 1:

Input: a = 1, b = 2
Output: "abb"
Explanation: "abb", "bab" and "bba" are all correct answers.

Example 2:

Input: a = 4, b = 1
Output: "aabaa"

Constraints:

• 0 <= a, b <= 100
• It is guaranteed such an s exists for the given a and b.

analysis

The meaning of the question is: given the number of a, the number of b, to form a string, the number of consecutive a or consecutive b should not exceed 3.

• I referred to the answer, and intuitively asked for an explanation, choosing the most letters left to write into the string. For example, if a = 6, b = 2, then we expect to write'aabaabaa'
• Use the flag to identify the variable to control the letter that currently needs to be selected. If the flag is True, select'a', otherwise select'b'
• The case of choosing'a' is that the length of a is greater than b, or the last two consecutive characters in the res result are'b', otherwise you need to choose'a'

Code

class Solution:
    def strWithout3a3b(self, a: int, b: int) -> str:
        res=''
        flag=False
        while(a>0 or b>0):
            if(len(res)>=2 and res[-1]==res[-2]):
                if(res[-1]=='b'):
                    flag=True
                else:
                    flag=False
            else:
                if(a>b):
                    flag=True
                else:
                    flag=False
            
            if(flag):
                res+='a'
                a-=1
            else:
                res+='b'
                b-=1
        return res

references

String without AAA or BBB