再维护一个iterator ，用来实现peek
/*
* Below is the interface for Iterator, which is already defined for you.
* **DO NOT** modify the interface for Iterator.
*
* class Iterator {
* struct Data;
* Data* data;
* Iterator(const vector<int>& nums);
* Iterator(const Iterator& iter);
*
* // Returns the next element in the iteration.
* int next();
*
* // Returns true if the iteration has more elements.
* bool hasNext() const;
* };
*/
class PeekingIterator : public Iterator {
public:
Iterator* iter;
int _next;
PeekingIterator(const vector<int>& nums) : Iterator(nums) {
// Initialize any member here.
// **DO NOT** save a copy of nums and manipulate it directly.
// You should only use the Iterator interface methods.
iter = new Iterator(nums);
if(nums.size()!=0)
_next = nums[0];
if(iter->hasNext())
iter->next();
}
// Returns the next element in the iteration without advancing the iterator.
int peek() {
return _next;
}
// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
int next() {
if(iter->hasNext())
_next = iter->next();
return Iterator::next();
}
bool hasNext() const {
return Iterator::hasNext();
}
};