341. Flatten Nested List Iterator

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