Find First Occurrence of an Element in a Sorted Array
Using Binary Search

public class FirstOccurrenceFinder {
  public static int solve(int n, int key, int[] v) {
    int start = 0;
    int end = n - 1;
    int res = -1;

    while (start <= end) {
      int mid = start + (end - start) / 2;
      if (v[mid] == key) {
        res = mid;       // Record the index
        end = mid - 1;   // Move left to check if it occurs earlier
      } else if (key < v[mid]) {
        end = mid - 1;
      } else {
        start = mid + 1;
      }
    }
    return res;
  }

  public static void main(String[] args) {
    int[] arr = {5, 10, 10, 10, 20, 20};
    int key = 10;
    int result = solve(arr.length, key, arr);
    System.out.println("First Occurrence Index: " + result);
  }
}