Google – Non Null Iterator


Google – Non Null Iterator
Input一个包含Null的iterator,wrap成一个Non-Null Iterator
[Solution]
Maintain a 'top' instance variable.
hasNext()的时候如果top不为null, 返回true。否则找到下一个不为null的为止,如果没有下一个了就返回false。
next()的时候直接返回top。但是这里要注意重置top为null,以便下一个hasNext()。
上面方法只要O(n)time和O(1)space。当然还有一种O(n) space的解法,把所有non-null element都存到一个list里面。

class Solution <T> implements Iterator<T> {
 
  Iterator<T> it;
  T top;
 
  public Solution(Iterator<T> it) {
    this.it = it;
  }
 
  @Override
  public T next() {
    T result = top;
    top = null;
    return result;
  }
 
  @Override
  public boolean hasNext() {
    if (top != null) {
      return true;
    }
 
    T curr = null;
    while (it.hasNext() && curr == null) {
      curr = it.next();
    }
    top = curr;
 
    return top != null;
  }
}
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