Medium
Design an iterator that supports the peek
operation on an existing iterator in addition to the hasNext
and the next
operations.
Implement the PeekingIterator
class:
PeekingIterator(Iterator<int> nums)
Initializes the object with the given integer iterator iterator
.int next()
Returns the next element in the array and moves the pointer to the next element.boolean hasNext()
Returns true
if there are still elements in the array.int peek()
Returns the next element in the array without moving the pointer.Note: Each language may have a different implementation of the constructor and Iterator
, but they all support the int next()
and boolean hasNext()
functions.
Example 1:
Input
["PeekingIterator", "next", "peek", "next", "next", "hasNext"]
[[[1, 2, 3]], [], [], [], [], []]
Output: [null, 1, 2, 2, 3, false]
Explanation:
PeekingIterator peekingIterator = new PeekingIterator([1, 2, 3]); // [1,2,3]
peekingIterator.next(); // return 1, the pointer moves to the next element [1,2,3].
peekingIterator.peek(); // return 2, the pointer does not move [1,2,3].
peekingIterator.next(); // return 2, the pointer moves to the next element [1,2,3]
peekingIterator.next(); // return 3, the pointer moves to the next element [1,2,3]
peekingIterator.hasNext(); // return False
Constraints:
1 <= nums.length <= 1000
1 <= nums[i] <= 1000
next
and peek
are valid.1000
calls will be made to next
, hasNext
, and peek
.Follow up: How would you extend your design to be generic and work with all types, not just integer?
import java.util.Iterator;
// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
public class PeekingIterator implements Iterator<Integer> {
private Iterator<Integer> it;
private Integer current;
public PeekingIterator(Iterator<Integer> iterator) {
// initialize any member here.
it = iterator;
current = it.next();
}
// Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
if (current == null) {
current = it.next();
}
return current;
}
// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
Integer temp = current;
current = it.hasNext() ? it.next() : null;
return temp;
}
@Override
public boolean hasNext() {
return current != null;
}
}