Java:实现String方法contains()没有内置方法contains()

Question

我试图在String method contains()不使用内置contains()方法的情况下实现.

这是我到目前为止:

public static boolean containsCS(String str, CharSequence cs) {

    char[] chs = str.toCharArray();
    int i=0,j=chs.length-1,k=0,l=cs.length();

    //String      str = "Hello Java";
    //                   0123456789
    //CharSequence cs = "llo";

    while(i<j) {
        if(str.charAt(i)!=cs.charAt(k)) {
            i++;
        }
        if(str.charAt(i)==cs.charAt(k)) {

        }
    }

    return false;
}

我只是练习我的算法技能并陷入困境.

有什么建议？

Answer 1

仅使用1个循环

我对Poran的答案做了一些补充,它完全正常:

 public static boolean contains(String main, String Substring) {
    boolean flag=false;
    if(main==null && main.trim().equals("")) {
        return flag;
    }
    if(Substring==null) {
        return flag;
    }

    char fullstring[]=main.toCharArray();
    char sub[]=Substring.toCharArray();
    int counter=0;
    if(sub.length==0) {
        flag=true;
        return flag;
    }

    for(int i=0;i<fullstring.length;i++) {

        if(fullstring[i]==sub[counter]) {
            counter++;
        } else {
            counter=0;
        }

        if(counter==sub.length) {
            flag=true;
            return flag;
        }

    }
    return flag;

}

Answer 2

正如 JB Nizet 所建议的，以下是实际代码contains()：

2123  public boolean contains(CharSequence s) {
2124      return indexOf(s.toString()) > -1;
2125  }

这是代码indexOf()：

1732     public int indexOf(String str) {
1733         return indexOf(str, 0);
1734     }

这导致：

 1752   public int indexOf(String str, int fromIndex) {
 1753       return indexOf(value, offset, count,
 1754                      str.value, str.offset, str.count, fromIndex);
 1755   }

最终导致：

 1770   static int indexOf(char[] source, int sourceOffset, int sourceCount,
 1771                      char[] target, int targetOffset, int targetCount,
 1772                      int fromIndex) {
 1773       if (fromIndex >= sourceCount) {
 1774           return (targetCount == 0 ? sourceCount : -1);
 1775       }
 1776       if (fromIndex < 0) {
 1777           fromIndex = 0;
 1778       }
 1779       if (targetCount == 0) {
 1780           return fromIndex;
 1781       }
 1782   
 1783       char first  = target[targetOffset];
 1784       int max = sourceOffset + (sourceCount - targetCount);
 1785   
 1786       for (int i = sourceOffset + fromIndex; i <= max; i++) {
 1787           /* Look for first character. */
 1788           if (source[i] != first) {
 1789               while (++i <= max && source[i] != first);
 1790           }
 1791   
 1792           /* Found first character, now look at the rest of v2 */
 1793           if (i <= max) {
 1794               int j = i + 1;
 1795               int end = j + targetCount - 1;
 1796               for (int k = targetOffset + 1; j < end && source[j] ==
 1797                        target[k]; j++, k++);
 1798   
 1799               if (j == end) {
 1800                   /* Found whole string. */
 1801                   return i - sourceOffset;
 1802               }
 1803           }
 1804       }
 1805       return -1;
 1806   }