NestedIterator.java

package design;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * Created by gouthamvidyapradhan on 30/11/2017.
 *
 * 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:
 Given the list [[1,1],2,[1,1]],

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

 Example 2:
 Given the list [1,[4,[6]]],

 By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].
 */
public class NestedIterator implements Iterator<Integer>{

    private List<Integer> result;
    private int curr, size;

    // This is the interface that allows for creating nested lists.
    // You should not implement it, or speculate about its implementation
    public interface NestedInteger{
        //@return true if this NestedInteger holds a single integer, rather than a nested list.
        public boolean isInteger();

        // @return the single integer that this NestedInteger holds, if it holds a single integer
        // Return null if this NestedInteger holds a nested list
        public Integer getInteger();

        // @return the nested list that this NestedInteger holds, if it holds a nested list
        // Return null if this NestedInteger holds a single integer
        public List<NestedInteger> getList();
    }

    public NestedIterator(List<NestedInteger> nestedList) {
        this.result = new ArrayList<>();
        curr = 0;
        flatten(result, nestedList);
        size = result.size();
    }

    @Override
    public Integer next() {
        if(curr < size){
            return result.get(curr++);
        }
        return -1;
    }

    @Override
    public boolean hasNext() {
        return curr < size;
    }

    public static void main(String[] args) {

    }

    private void flatten(List<Integer> flatList, List<NestedInteger> nestedList){
        for(NestedInteger n : nestedList){
            if(n.isInteger()){
                flatList.add(n.getInteger());
            } else{
                flatten(flatList, n.getList());
            }
        }
    }

}