【LeetCode】初级算法案例+java代码实现（数学篇）

@TOC

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# 一、Fizz Buzz  ```java public List fizzBuzz(int n) { List list = new ArrayList<>(n); for (int i = 1; i <= n ; i++) { boolean b1 = i%3==0; boolean b2 = i%5==0; if(b1&&b2){ list.add("FizzBuzz"); }else if(b1){ list.add("Fizz"); }else if(b2){ list.add("Buzz"); }else{ list.add(""+i); } } return list; } ```

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# 二、计数质数   ```java public int countPrimes(int n) { boolean[] arr = new boolean[n]; int cnt = 0; for(int i = 2; i < n; i++) { if(arr[i]) continue; cnt++; for(int j = i; j < n; j+=i) { arr[j] = true; } } return cnt; } ```

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# 三、3的幂  ```java public boolean isPowerOfThree(int n) { if (n > 1){ while (n % 3 == 0){ n /= 3; } } return n == 1; } ```

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# 四、罗马数字转整数   ```java public int romanToInt(String s) { Map map = new HashMap<>(); map.put("I",1); map.put("IV",4); map.put("V",5); map.put("IX",9); map.put("X",10); map.put("XL",40); map.put("L",50); map.put("XC",90); map.put("C",100); map.put("CD",400); map.put("D",500); map.put("CM",900); map.put("M",1000); if(map.containsKey(s)){ return map.get(s); }else{ StringBuilder sb = new StringBuilder(); int num = 0; char[] chars = s.toCharArray(); for (int i = 0; i < chars.length; i++) { if(!map.containsKey(sb.toString()+chars[i])){ if(sb.length()>0){ num += map.get(sb.toString()); sb = new StringBuilder(); i--; } }else{ sb.append(chars[i]); } } if(sb.length()>0){ num += map.get(sb.toString()); } return num; } } ```

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