LeetCode 341. Flatten Nested List Iterator

/** * // 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> {Stack<NestedInteger> stack = new Stack<NestedInteger>();    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 ni = stack.peek();        if (ni.isInteger()) return true;        stack.pop();        List<NestedInteger> nestedList = ni.getList();        for (int i = nestedList.size() - 1; i >= 0; i--) stack.push(nestedList.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(); */