341. Flatten Nested List Iterator

• Difficulty: Medium

• Topics: Stack, Design

• Similar Questions:

  • Flatten 2D Vector

  • Zigzag Iterator

  • Mini Parser

  • Array Nesting

Problem:

Given a nested list of integers, implement an iterator to flatten it.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1:

Input: [[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].

Input: [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].

Solutions:

/**
 * // 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) {
        for (int i = 0; i < nestedList.size(); ++i) {
            stk.push(nestedList[nestedList.size() - 1 - i]);
        }
    }

    int next() {
        int val = stk.top().getInteger();
        stk.pop();
        return val;
    }

    bool hasNext() {
        while (!stk.empty()) {
            if (stk.top().isInteger()) {
                return true;
            } else {
                auto nestedList = stk.top().getList(); stk.pop();
                for (int i = 0; i < nestedList.size(); ++i) {
                    stk.push(nestedList[nestedList.size() - 1 - i]);
                }
            }
        }
        return false;
    }

private:
    stack<NestedInteger> stk;
    int buffer;
};

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