420. Strong Password Checker

这道题要分成几种情况做
When len > 20, we need to do len - 20 times deletion. Also we need to do a change for every three repeating characters.

For any repeating sequences with len % 3 == 0, we can reduce one replacement by deleting one character. For any repeating sequences with len % 3 == 1, we can reduce one replacement by deleting two character. For the remaining sequences, we can reduce every replacement by deleting three character.

class Solution(object):
    def strongPasswordChecker(self, s):
        """
        :type s: str
        :rtype: int
        """
        missing_type = 3
        if any('a' <= c <= 'z' for c in s): missing_type -= 1
        if any('A' <= c <= 'Z' for c in s): missing_type -= 1
        if any(c.isdigit() for c in s): missing_type -= 1
            
        change = 0
        one = two = 0
        p = 2
        while p < len(s):
            if s[p] == s[p-1] == s[p-2]:
                length = 2
                while p < len(s) and s[p] == s[p-1]:
                    length += 1
                    p += 1
                    
                change += length / 3
                if length % 3 == 0: one += 1
                elif length % 3 == 1: two += 1
            else:
                p += 1
        
        if len(s) < 6: 
            return max(missing_type, 6 - len(s))
        elif len(s) <= 20:
            return max(missing_type, change)
        else:
            delete = len(s) - 20
            # change表示多少个replacement需要做
            # 因为需要删除，所以replacement可以用删除来代替
            # 对于aaa这种可以用删除a来代替replace a的操作
            # 对于aaaa这种可以删除aa来代替replace a的操作
            # 对于aaaaaa这种可以删除aaa来代替replace a的操作
            change -= min(delete, one) 
            change -= min(max(delete - one, 0), two * 2) / 2
            change -= max(delete - one - 2 * two, 0) / 3
                
            return delete + max(missing_type, change)