Coding refresher on Github

It gives an immense pleasure to share my technical experience with fellow people. I started pushing all my local code fragments/programs to github as a way of helping young/new programmers on how to code in Java.

Github link to my program list that helped me to attain "Oracle Certified Java Professional".

https://github.com/athiruban/CodingRefresher

Many new interesting topics on DataStructure and Algorithms will be added to the above repo.

Please visit and share your comments on how we can improve this further.

Largest rectangle in a histogram

The below code finds the rectangle with maximum area in a histogram with O(n) time complexity.

    public static void maxRectArea(int a[], int b[]) {
        // this is a maximization problem
        // irrespective of the height of the the next bar
        // we have to compare the area if including next bar exceeds the current area then we have to include the bar.
        // stop the expansion and proceed further until end of input.
        // 6 2 5 4 5  1 6
        // 6 2 5 8 12 1 6
        int i = 0;
        int max = -1;
        int e = 0;

        while (i < a.length) {
            if (i == 0)
                e = b[i] = a[i];
            else {
                if (a[i] < a[i-1]) {
                    int t = b[i-1] / e;
                    if (((t+1) * a[i]) < b[i-1])
                        e = b[i] = a[i];
                    else {
                        e = a[i];
                        b[i] = (t+1) * a[i];    
                    }
                }
                else if (a[i] == a[i-1]) {
                    b[i] = b[i-1] + e;
                }
                else if (a[i] > a[i-1]) {
                    if ((b[i-1] + e) > a[i]) {
                        b[i] = b[i-1] + e;
                    }
                    else {
                        e = b[i] = a[i];
                    }
                }         
            }
            i++;
        }
    }

Using recursion

//Substring using recursion

substring("", userinput, 0, userinput.length());

public static void substring(String fixed, String str, int step, int len) {

     if (step < len) {

       System.out.printf("%d :%s \n", ++counter, fixed);

       while (step < len) {

         substring(fixed + str.charAt(step), str, step + 1, len);

         step ++;

       }
     }

     else

     System.out.printf("%d :%s \n", ++counter, fixed);

   }

//Anagram using recursion

  public static void anagram(String str, int len) {

    anagram0("", str, len);

  }

  public static void anagram0(String first, String str,

                              int len) {

    String left = "", right = "";

    if (len == 1) {

      System.out.println(":" + first + str);

    }

    else {

      for (int pos = 0; pos < len; pos++) {

        String temp = "" + str.charAt(pos);

        left = str.substring(0, pos);

        if (pos + 1 == len)

          right = "";

        else

          right = str.substring(pos + 1, len);

        anagram0(first + temp, left + right, len - 1);

      }

    }

  }