341.flatten-nested-list-iterator.cpp

//  Tag: Stack, Tree, Depth-First Search, Design, Queue, Iterator
//  Time: O(*1)
//  Space: O(N)
//  Ref: -
//  Note: -

//  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].
//  
//   
//  Constraints:
//  
//  1 <= nestedList.length <= 500
//  The values of the integers in the nested list is in the range [-106, 106].
//  
//  

/**
 * // 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:
    stack<NestedInteger> st;
    NestedIterator(vector<NestedInteger> &nestedList) {
        for (int i = nestedList.size() - 1; i >= 0; i--) {
            st.push(nestedList[i]);
        }
    }
    
    int next() {
        NestedInteger top = st.top();
        st.pop();
        return top.getInteger();
    }
    
    bool hasNext() {
        while (!st.empty()) {
            NestedInteger cur = st.top();
            if (cur.isInteger()) {
                return true;
            } else {
                st.pop();
                auto &list = cur.getList();
                for (int i = list.size() - 1; i >= 0; i--){
                    st.push(list[i]);
                }
            }
        }
        return false;
    }
};

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