我开始想错了。
    正确解法应该是创建一个Stack<NestedInteger> stack把所有的元素都放进去，然后不断的pop()

    • 如果是Integer，就返回
    • 如果是NestedInteger，就再把这个的List给push()到stack中

    代码如下：

    1. /**
    2.  * // This is the interface that allows for creating nested lists.
    3.  * // You should not implement it, or speculate about its implementation
    4.  * public interface NestedInteger {
    5.  *
    6.  *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
    7.  *     public boolean isInteger();
    8.  *
    9.  *     // @return the single integer that this NestedInteger holds, if it holds a single integer
    10.  *     // Return null if this NestedInteger holds a nested list
    11.  *     public Integer getInteger();
    12.  *
    13.  *     // @return the nested list that this NestedInteger holds, if it holds a nested list
    14.  *     // Return null if this NestedInteger holds a single integer
    15.  *     public List<NestedInteger> getList();
    16.  * }
    17.  */
    18. public class NestedIterator implements Iterator<Integer> {
    19.     Stack<NestedInteger> stack = new Stack<>();
    20.     public NestedIterator(List<NestedInteger> nestedList) {
    21.         pushHelper(nestedList);
    22.     }
    23.     @Override
    24.     public Integer next() {
    25.         return stack.pop().getInteger();
    26.     }
    27.     @Override
    28.     public boolean hasNext() {
    29.         while (!stack.empty()) {
    30.             NestedInteger tmp = stack.peek();
    31.             if (tmp.isInteger()) {
    32.                 break;
    33.             }
    34.             else {
    35.                 stack.pop();
    36.                 pushHelper(tmp.getList());
    37.             }
    38.         }
    39.         return !stack.empty();
    40.     }
    41.     private void pushHelper(List<NestedInteger> nestedList) {
    42.         for (int i = nestedList.size() - 1; i >= 0; --i) {
    43.             stack.push(nestedList.get(i));
    44.         }
    45.     }
    46. }
    47. /**
    48.  * Your NestedIterator object will be instantiated and called as such:
    49.  * NestedIterator i = new NestedIterator(nestedList);
    50.  * while (i.hasNext()) v[f()] = i.next();
    51.  */