如何使用charAt()和length()编写是否为子字符串方法 [英] How to use charAt() and length() to write a whether is substring method

问题描述

我想编写一个 boolean 方法 subString()来判断字符串 s1 是否是 s2 .

I want to write a boolean method subString() to judge if string s1 is the substring of s2.

要求仅使用 charAt() 和 length() 以下方法/strong> 字符串.

例如

 Substring("abc","abcd")-> true

 Substring("at","cat")->true

 Substring("ac","abcd")->false

indexOf() 无法使用.

这是我到目前为止所得到的.

Here is what I got so far.

public class Q3 {
    public boolean subString(String str1, String str2) {
        String s1 = str1.toLowerCase();
        String s2 = str2.toLowerCase();
        for (i = 0; i < s1.length; i++) {
            for (j = 0; j < s2.length; j++) {
                if (s1.charAt(i) == s2.charAt(j))
                    return true;
            }
        }
        return false;
    }
}

测试类为:

public class Q3test {
    public static void main (String arg[]){
        Q3 Q3object = new Q3();
        System.out.println(Q3object.Substring("ac","abcd"));
    }
}

失败 subString("ac"，"abcd")，因为它返回true.

It fails subString("ac","abcd") as it returns true.

推荐答案

如果第一个字符匹配，您的代码将返回true.您需要将第一个String的所有字符包含在第二个String的子字符串中.

Your code returns true if the first character matches. You need all the characters of the first String to be contained in a substring of the second String.

我的原始代码有误.这是正确的代码:

My original code was wrong. Here's the correct code :

        public static boolean subString(String str1, String str2)
        {
          String s1 = str1.toLowerCase();
          String s2 = str2.toLowerCase();
          for (int offset = 0; offset <= s2.length() - s1.length(); offset++) {
            int i = 0;
            for (; i < s1.length(); i++){
              if(s1.charAt(i) != s2.charAt(i+offset)) {
                break;
              }
            }
            // found a substring that starts at the current offset
            if (i == s1.length())
              return true;
          }
          return false;
        }

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