[LeetCode]420. Strong Password Checker深入浅出算法讲解和代码示例

1、汇总概要

本题较简单，主要考虑时间复杂度问题，解题用到了哈希表。

2、题目

A password is considered strong if below conditions are all met:

1. It has at least 6 characters and at most 20 characters.
2. It must contain at least one lowercase letter, at least one uppercase letter, and at least one digit.
3. It must NOT contain three repeating characters in a row ("...aaa..." is weak, but "...aa...a..." is strong, assuming other conditions are met).

Write a function strongPasswordChecker(s), that takes a string s as input, and return the MINIMUM change required to make s a strong password. If s is already strong, return 0.

Insertion, deletion or replace of any one character are all considered as one change.

3、审题

给定一串密码，找出其最少需更改几处，以满足密码规则。

4、解题思路

本题的密码规则分别是密码串长度、含小写、大写、数字、不能有连续重复三个的字符，共5个规则。

为一次遍历密码串后，得到结果，即时间复杂度为O(N)，用哈希表记录满足规则的状态，以减少判断次数。

算法不复杂，直接上代码。

5、代码示例 - Java

import java.util.HashMap;

public class StrongPasswordChecker {
	public int PWDCheck(String pwd){ 
		String res = "";
		HashMap<String,Integer> hm = new HashMap<String,Integer>();
		int metNum = 0;
		int maxRepNum = 1;
		
		int len=pwd.length();
		if(len>=6 && len<=20){
			hm.put("isLength", 1);
			metNum++;
		}else{
			hm.put("isLength", 0);
		}
		for(int i=0;i<len;i++){ 
			char c = pwd.charAt(i);
			if(Character.isLowerCase(c)){
				if(!hm.containsKey("isLowercase")){
					hm.put("isLowercase", 1);
					metNum++;
				}
			}else if(Character.isUpperCase(c)){
				if(!hm.containsKey("isUppercase")){
					hm.put("isUppercase", 1);
					metNum++;
				}
			}else if(Character.isDigit(c)){
				if(!hm.containsKey("isDigit")){
					hm.put("isDigit", 1);
					metNum++;
				}
			}
			if(!hm.containsKey("isRepeat3")){
				if(i<len-1 && pwd.charAt(i) == pwd.charAt(i+1)){
					maxRepNum++;
					if(maxRepNum>=3){
						hm.put("isRepeat3", 1);
					}
				}else{
					maxRepNum = 1;
				}
			}
		}
		if(!hm.containsKey("isRepeat3")){
			metNum++;
		}
		
		int minimum = 5 - metNum; //get Minimum change
		return minimum;
	}
	public static void main(String[] args){
		String pwd = "aa11bcBBB";
		StrongPasswordChecker spwc = new StrongPasswordChecker();
		int minimum = spwc.PWDCheck(pwd);
		System.out.println("minimum: "+minimum);
	}
}

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