NestedIterator.java

/**
 * 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].
 *
 */

/**
 * // 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 class NestedIterator implements Iterator<Integer> {
    Queue<Integer> q = new LinkedList<>();

    public NestedIterator(List<NestedInteger> nestedList) {
        flattenNestedList(nestedList);
    }

    private void flattenNestedList(List<NestedInteger> nestedList) {
        for (NestedInteger ni: nestedList) {
            if (ni.isInteger()) {
                q.add(ni.getInteger());
            } else {
                flattenNestedList(ni.getList());
            }
        }
    }

    @Override
    public Integer next() {
        return q.remove();
    }

    @Override
    public boolean hasNext() {
        return !q.isEmpty();
    }
}

/*
public class NestedIterator implements Iterator<Integer> {
    Stack<NestedInteger> stack = new Stack<>();
    public NestedIterator(List<NestedInteger> nestedList) {
        for(int i = nestedList.size() - 1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }

    @Override
    public Integer next() {
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!stack.isEmpty()) {
            NestedInteger curr = stack.peek();
            if(curr.isInteger()) {
                return true;
            }
            stack.pop();
            for(int i = curr.getList().size() - 1; i >= 0; i--) {
                stack.push(curr.getList().get(i));
            }
        }
        return false;
    }
}
*/

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */