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> {
    Stack<NestedInteger> stack  = new Stack<>(); 

    public NestedIterator(List<NestedInteger> nestedList) {
        for (int i = nestedList.size()-1; i >= 0; i--) {
            //倒叙把所有的nestedList放在stack里面
            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; 
            } else {
                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();
 */
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