Java 面试手撕代码

1. 判断括号有效性

public static boolean fun5(String str) {
    HashMap<Character, Character> hashMap = new HashMap<>();
    hashMap.put(')', '(');
    hashMap.put(']', '[');
    hashMap.put('}', '{');

    LinkedList<Character> stack = new LinkedList<>();
    for (char c : str.toCharArray()) {
        if (hashMap.containsKey(c)) {
            if (stack.peek() != hashMap.get(c)) {
                return false;
            }
            stack.pop();
        } else {
            stack.push(c);
        }
    }
    return stack.isEmpty();
}

2. 输出字符串可能的组合次数

package com.example.helloworld;

import java.util.HashMap;

public class Main1
{
    public static void main (String[] args)
    {
        // 输出字符串可能的组合次数
        String str = "AABBCC";
        System.out.println(fun(str));
    }
    public static int fun(String str) {
        int count = 0;
        if (str == null || "".equals(str)) {
            return count;
        }
        HashMap<Character, Integer> hashMap = new HashMap<>();
        for (Character ch : str.toCharArray()) {
            if (hashMap.containsKey(ch)) {
                int a = hashMap.get(ch);
                hashMap.put(ch,++a);
            } else {
                hashMap.put(ch, 1);
            }
        }
        int len = calculate(str.length());
        int result = 1;
        for (Integer integer :hashMap.values()){
            result = calculate(integer) * result;
        }
        count = len / result;
        return count;
    }
    public static int calculate(int value) {
        if (value == 0) {
            return 0;
        }
        if (value == 1) {
            return 1;
        }
        return value * calculate(value - 1);
    }
}

3. 大小写转换

package com.example.helloworld;

import java.util.HashMap;
import java.util.Map;

public class Main
{
    public static void main (String[] args)
    {
        /**
         * 题目大意
         * 定义每个字母的位置：A->1 B->2 …Z->26… a->1 b->2 … z->26
         * 给一个只包含大小写字母的字符串，做以下操作
         * 1.将字符串升序排序
         * 2.对应一个字符，将其位置值的平方除26取余数+1得到新位置，转换成对应位置字符，大写的和大写的转，小写的和小写的转
         * 样例
         * 样例1:输入：bA  输出：Be
         * 解释：bA排序Ab，A对应1，(1 * 1) % 26 + 1 = 2，A转换成B, b对应2,(2 * 2) % 26 + 1 = 5,5对应e，所以最后输出Be
         * 样例2: 输入：HuaweiIsBest  输出：EMDbzzdxxkzj
         */
        String str= "HuaweiIsBest";
        System.out.println(fun(str));
    }
    public static String fun(String str) {
        HashMap<Character, Integer> hm1 = new HashMap<>();
        hm1.put('A', 1);
        hm1.put('B', 2);
        hm1.put('C', 3);
        hm1.put('D', 4);
        hm1.put('E', 5);
        hm1.put('F', 6);
        hm1.put('G', 7);
        hm1.put('H', 8);
        hm1.put('I', 9);
        hm1.put('J', 10);
        hm1.put('K', 11);
        hm1.put('L', 12);
        hm1.put('M', 13);
        hm1.put('N', 14);
        hm1.put('O', 15);
        hm1.put('P', 16);
        hm1.put('Q', 17);
        hm1.put('R', 18);
        hm1.put('S', 19);
        hm1.put('T', 20);
        hm1.put('U', 21);
        hm1.put('V', 22);
        hm1.put('W', 23);
        hm1.put('X', 24);
        hm1.put('Y', 25);
        hm1.put('Z', 26);
        HashMap<Character, Integer> hm2 = new HashMap<>();
        hm2.put('a', 1);
        hm2.put('b', 2);
        hm2.put('c', 3);
        hm2.put('d', 4);
        hm2.put('e', 5);
        hm2.put('f', 6);
        hm2.put('g', 7);
        hm2.put('h', 8);
        hm2.put('i', 9);
        hm2.put('j', 10);
        hm2.put('k', 11);
        hm2.put('l', 12);
        hm2.put('m', 13);
        hm2.put('n', 14);
        hm2.put('o', 15);
        hm2.put('p', 16);
        hm2.put('q', 17);
        hm2.put('r', 18);
        hm2.put('s', 19);
        hm2.put('t', 20);
        hm2.put('u', 21);
        hm2.put('v', 22);
        hm2.put('w', 23);
        hm2.put('x', 24);
        hm2.put('y', 25);
        hm2.put('z', 26);
        char[]  chars= str.toCharArray();
        for (int i = 0; i < chars.length; i++) {
            for (int j = i + 1; j < chars.length; j++) {
                if (chars[i] > chars[j]) {
                    char t = chars[i];
                    chars[i] = chars[j];
                    chars[j] = t;
                }
            }
        }
        //将其位置值的平方除26取余数+1得到新位置，转换成对应位置字符，大写的和大写的转，小写的和小写的转
        StringBuilder sb = new StringBuilder();
        for (Character ch : chars) {
            if (hm1.containsKey(ch)) {
                int v = hm1.get(ch);
                int newInt = calculate(v);
                for (Map.Entry<Character, Integer> entry : hm1.entrySet()) {
                    if (entry.getValue() == newInt) {
                        sb.append(entry.getKey());
                    }
                }
            } else if (hm2.containsKey(ch)) {
                int v = hm2.get(ch);
                int newInt = calculate(v);
                for (Map.Entry<Character, Integer> entry : hm2.entrySet()) {
                    if (entry.getValue() == newInt) {
                        sb.append(entry.getKey());
                    }
                }
            }
        }
        return sb.toString();
    }
    public static int calculate(int val){
        double v = Math.pow(val, 2) % (double)26;
        return (int)(Math.round(v) +1);
    }
}