Leetcode: 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].

非常精巧地使用stack。push all the nestedList into the stack from back to front，so when we pop the stack, it returns the very first element

执行hasNext()的时候，如果peek()是integer, return true；否则是一个List<NestedInteger>, 则flatten这个list，同样的，把list里的NestedInteger倒序push入栈

 /**
  * // 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> st;

     public NestedIterator(List<NestedInteger> nestedList) {
         st = new Stack<NestedInteger>();
         for (int i=nestedList.size()-1; i>=0; i--) {
             st.push(nestedList.get(i));
         }
     }

     @Override
     public Integer next() {
         return st.pop().getInteger();
     }

     @Override
     public boolean hasNext() {
         while (!st.isEmpty()) {
             if (st.peek().isInteger()) return true;
             List<NestedInteger> cur = st.pop().getList();
             for (int i=cur.size()-1; i>=0; i--) {
                 st.push(cur.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();
  */