341. Flatten Nested List Iterator

https://leetcode.com/problems/flatten-nested-list-iterator/

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].

Example 2:

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].

Solution

1. Recursive call to flatten an array

/**
 * // 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 {
private:
    vector<int> flattened;
    int current_index;
    void Flatten(const vector<NestedInteger> &nestedList){
        for (const auto& n: nestedList) {
            if (n.isInteger()) flattened.push_back(n.getInteger());
            else Flatten(n.getList());
        }
    }
public:
    NestedIterator(vector<NestedInteger> &nestedList): current_index(0) {
        Flatten(nestedList);
    }
    
    int next() {
        return flattened[current_index++];
    }
    
    bool hasNext() {
        return current_index < flattened.size();
    }
};

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

2. Using stack

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

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

• #dfs