Leetcode 17. Letter Combinations of a Phone Number(python)

题目

解析

这题实际上是一个无重复combination的模板题，只需要对1做一下特殊处理即可
关于combination和permutation的总结，看这里：

代码：

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        def backtracking(comb,level):
            if len(comb)==length:
                ans.append(comb[:])
            
            for i in range(level,len(digits)):
                if digits[level]=='1':
                    continue
                for c in d[digits[i]]:
                    #comb.appned(c)
                    backtracking(comb+c,i+1)
                    #comb.pop()
                    
        
        count_1 = 0
        for digit in digits:
            if digit == '1':
                count_1 += 1
        
        d = {'2':['a','b','c'],'3':['d','e','f'],'4':['g','h','i'],'5':['j','k','l'],'6':['m','n','o'],'7':['p','q','r','s'],'8':['t','u','v'],'9':['w','x','y','z']}
        if not digits:
            return []
        ans = []
        length = len(digits)-count_1
        backtracking('',0)
        return ans