2022-12-22 leetcode 1415 如果用

class Solution:
    def getHappyString(self, n: int, k: int) -> str:
        if k>3 * (1 << (n - 1)):  #大侠教的
            return ''
        self.a=self.f=0
        self.ss=''
        def dfs(s):
            if self.f:  #开关
                return 
            if len(s)==n:
                self.a+=1 #叔叔数数
                if self.a==k:   #找到了
                    self.f=1
                    self.ss=s
                return

            for c in 'abc':
                if s and c==s[-1]:
                    continue
                t=s+c   
                dfs(t)  
        dfs('')
        return self.ss