LeetCode.341 Flatten Nested List Iterator

题目：

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].

分析：

/** * // 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> {    //考虑到子集中还有集合，当遍历到子集为集合时，需要使用stack。然后递归    Stack<NestedInteger> stack;        public NestedIterator(List<NestedInteger> nestedList) {        stack=new Stack<NestedInteger>();        for(int i=nestedList.size()-1;i>=0;i--){            //倒序将整个子集放入           stack.push(nestedList.get(i));         }            }    @Override    public Integer next() {         //从stack中取出元素        return stack.pop().getInteger();            }    @Override    public boolean hasNext() {        while(!stack.empty()){            NestedInteger cur=stack.peek();            if(cur.isInteger()){                return true;            }            //那么就是空串            stack.pop();            //不是空串的集合            for(int i=cur.getList().size()-1;i>=0;i--){                stack.push(cur.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(); */