341_flatten_nested_list_iterator.cpp

//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.

//Your code will be tested with the following pseudocode:

//initialize iterator with nestedList
//res = []
//while iterator.hasNext()
//    append iterator.next() to the end of res
//return res

//If res matches the expected flattened list, then your code will be judged as correct.

//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].

/**
 * // 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;
 * };
 */
/**
 * // 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 {
    vector<int> flattenedList;
    vector<int>::iterator it;

    void flat(vector<NestedInteger> &nestedList) {
        for (NestedInteger &ni : nestedList) {
            if (ni.isInteger())
                flattenedList.push_back(ni.getInteger());
            else
                flat(ni.getList());
        }
    }

public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        flat(nestedList);
        it = flattenedList.begin();        
    }
    
    int next() {
        return *it++;
    }
    
    bool hasNext() {
        return it != flattenedList.end();
    }
};

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

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