leetcode 284. Peeking Iterator（顶端迭代器）

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 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
All the calls to next and peek are valid.
At most 1000 calls will be made to next, hasNext, and peek.

模拟iterator，peek是检索当前iterator指向的元素，next是完成了peek的功能之后再把指针指向下一个元素。

思路：
先找个list把元素都装起来，再利用下标。
因为arrayList在add元素时，扩容会增加计算开销，所以要提前指定容量。

因为题目中说next和peek都是有效的，所以直接调用即可。

class PeekingIterator implements Iterator<Integer> {
    List<Integer> list = new ArrayList<>(1001);
    int id = 0;
    
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
        while(iterator.hasNext()) {
            list.add(iterator.next());
        }
	}
	
    // Returns the next element in the iteration without advancing the iterator.
	public Integer peek() {        
        return list.get(id);
	}
	
	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	@Override
	public Integer next() {
	    return list.get(id ++);
	}
	
	@Override
	public boolean hasNext() {
	    return id < list.size();
	}
}