Question

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

/**
 * // 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> {
    public NestedIterator(List<NestedInteger> nestedList) {
        lists = new Stack<>();
        lists.push(nestedList.listIterator());
        verified = false;
    }

    public Integer next() {
        if(!verified)
            hasNext();
        verified = false;
        return lists.peek().next().getInteger();
    }

    public boolean hasNext() {
        if(verified) return true;
        while (!lists.empty()) {
            if (!lists.peek().hasNext()) {
                lists.pop();
            } else {
                NestedInteger x = lists.peek().next();
                if (x.isInteger()) {
                    lists.peek().previous();
                    verified = true;
                    return true;
                } else
                    lists.push(x.getList().listIterator());
            }
        }
        return false;
    }

    private Stack<ListIterator<NestedInteger>> lists;
    private boolean verified;
}

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