CareerCup

public void replaceCharacter(String s1, String s2, char c) {

StringBuilder builder = new StringBuilder( "", s1.length);

foreach(char toCheck in s1) {
	if( (int) c == (int) toCheck) {
		builder.append(s2)
	} else {
		builder.append(toCheck);
	}
}

If you cannot even use .length then I would do it inside a while loop and not a for loop. To improve the big O notation you can use recursing while dividing the array I guess. Not too sure about that.

couldn't find any other way of reading char from console..

public class StringReplacement {

    public static void main(String[] args){

        Scanner scanner=new Scanner(System.in);

        char c=scanner.next().charAt(0);
        String str1= scanner.next();
        String str2= scanner.next();
        String newstr=new String();

        for(char ch:str1.toCharArray()){

            if(ch==c){
              newstr=newstr+str2;
            }

            else{
                newstr=newstr+ch;
            }
        }

        System.out.println(newstr);
    }
}

void find_and_replace(char **S1, const char ch, const char* S2)
{
	char* str1 = strdup(*S1);
	int m=0;
	while(str1[m] != '\0')
	{
		if(str1[m] != ch)
		{
			m++;
		}
		else
		{
			str1 = realloc(str1, strlen(str1) + strlen(S2));
			memcpy(str1 + m + strlen(S2), str1 + m + 1, strlen(str1 + m + 1));
			strncpy(str1 + m, S2, strlen(S2));
			m += strlen(S2);
		}
	}
	*S1 = str1;
}

int main()
{
	char* S1 = "this is a test";
	char* S2 = "blah";
	char ch = 't';
	printf("%s\n", S1);
	find_and_replace(&S1, ch, S2);
	printf("%s\n", S1);
	return 0;
}

Represent the Strings as char[], scan the first string once, to count the number of times that char c is present, now calculate the length of the final string, and fill it from backwards O(2n) = O(n)

Example: abcdcdef and xyz are 2 strings, represent them as a char[], given char c, now scan first string, we find c twice, so the final string length is 8 - 1 + 3 - 1 + 3 = 12, create a new char[] and fill it from backwards with the first string, but when u hit c, fill it with the second string xyz instead of the c.

public class ReplaceCharFromString{

     public static void main(String []args){
         String str1 = "stauvwamna";
         char[] arr1 = str1.toCharArray();
         char str2 = 'X';
         char chr = 'a';
         for(int i=0;i<arr1.length;i++){
             if(arr1[i]==chr){
                  arr1[i]=str2;
                  System.out.println(arr1[i]);
             }else
             System.out.println(arr1[i]);
             
         }
     }
}

public class Findandreplace {
public static void main(String args[]){
String name = "napendra";
String ch = "a";
String name1 = "SINGH";
String newName = name.replaceAll(ch, name1);
System.out.println(newName);
}

}

public static String insertString(String subject, String inserter, char c1) {
String result = subject;
char[] array = result.toCharArray();
for (int i = 0; i < array.length; i++ ) {
if (array[i] == c1) {
result = new StringBuilder(result).replace(i, i+1, inserter).toString();
i+= (inserter.length()-1);
array = result.toCharArray();
}
}
return result;
}