Add two pointer algorithm

[pathikrit]

  /**
    * Two pointer algorithm
    * O(2*n)
    * See:
    *   http://codeforces.com/contest/676/problem/C
    *   http://codeforces.com/contest/580/problem/B
    *
    * @param n maximum right bound
    * @param f given an interval returns if it is acceptable
    *        f must be monotonic i.e. for all super-interval j of i, f(j) implies f(i)
    *
    * @return List of all intervals in [0, n) at which f is acceptable
    */
  def validIntervals(n: Int)(f: (Int, Int) => Boolean): Iterator[(Int, Int)] = {
    val it = new Iterator[Option[(Int, Int)]] {
      var l, r = 0
      override def hasNext = r < n
      override def next() = {
        if(f(l, r)) {
          r += 1
          Some((l, r - 1))
        } else {
          if(l < r) l += 1 else r += 1
          None
        }
      }
    }
    it.flatten
  }

[pathikrit]

See: https://tp-iiita.quora.com/The-Two-Pointer-Algorithm