字符串及其反转中是否存在同一子字符串

class Solution {
    public boolean isSubstringPresent(String s) {
 StringBuilder sb = new StringBuilder(s);
    String reverse = sb.reverse().toString(); 
    for (int i = 0; i <= s.length() - 2; i++) { // 修改循环终止条件为 <=，以便遍历到倒数第二个字符
        if (reverse.contains(s.substring(i, i + 2))) { // 提取子字符串时范围要包括 i 到 i+1
            return true;
        }
    }
    return false;
    }
}

 统计以给定字符开头和结尾的子字符串总数

class Solution {
  public long countSubstrings(String s, char c) {
        long ans = 0;
        char[]chars = s.toCharArray();
        long k =1;
        for(int i=0;i<s.length();i++){
            if(chars[i]==c){
                ans=(long)ans+k;
                k++;
            }
        }
        return ans;
    }
}

成为 K 特殊字符串需要删除的最少字符数

 

class Solution {
    public int minimumDeletions(String word, int k) {
     Map<Character,Integer>map = new HashMap<>();
            char []chars = word.toCharArray();
            Character minCharater = 0;
            int v = -1;
            int sum = Integer.MAX_VALUE;
            for(int i=0;i<word.length();i++ ){
                map.putIfAbsent(chars[i],0);
                map.put(chars[i],map.get(chars[i])+1);
            }
            if(map.size()==1||map.size()==0)return 0;

            List<Integer> list = new ArrayList<>();
            for(Map.Entry<Character,Integer>entry:map.entrySet()){
                list.add(entry.getValue());
            }
            Collections.sort(list);
            int m = list.size();
            Collections.sort(list);

            for(int i = 0; i < m ; i ++){
                int temp = -1;
                for(int j = 0; j < m ; j ++){
                    if(i > j){
                        temp += list.get(j);
                    }else if(i == j){
                        continue;
                    }else{
                        if(list.get(j) - list.get(i) <= k){
                            continue;
                        }else{
                            temp += list.get(j) - k - list.get(i);
                        }
                    }
                }
                // System.out.println(temp + 1);
                // if(temp == -1)continue;
                sum = Math.min(sum,temp + 1);
            }
            return sum;
    }
}