[LeetCode April Challange] Day 13 - Flatten Nested List Iterator

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Example 1:

Input: nestedList = [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

Input: nestedList = [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Constraints:

• 1 <= nestedList.length <= 500
• The values of the integers in the nested list is in the range [-10^6, 10^6].

---

Solution

/**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* class NestedInteger {
*   public:
*     // Return true if this NestedInteger holds a single integer, rather than a nested list.
*     bool isInteger() const;
*
*     // Return the single integer that this NestedInteger holds, if it holds a single integer
*     // The result is undefined if this NestedInteger holds a nested list
*     int getInteger() const;
*
*     // Return the nested list that this NestedInteger holds, if it holds a nested list
*     // The result is undefined if this NestedInteger holds a single integer
*     const vector<NestedInteger> &getList() const;
* };
*/

class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        begin.push(nestedList.begin());
        end.push(nestedList.end());
    }
    
    int next() {
        return hasNext() ? (begin.top()++)->getInteger() : -1;
    }
    
    bool hasNext() {
        while (!begin.empty()) {
            if (begin.top() == end.top()) {
                begin.pop();
                end.pop();
            } else {
                if (begin.top()->isInteger()) return true;
                end.push(begin.top()->getList().end());
                begin.push((begin.top()++)->getList().begin());
            }
        }
        return false;
    }
private:
    stack<vector<NestedInteger>::iterator> begin, end;
};

/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i(nestedList);
* while (i.hasNext()) cout << i.next();
*/

用 begin 和 end stack 來存放第 n level vector 的起終。
呼叫 hasNext() 時，會將 begin.top() 指向下一個Integer。