157.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].

分析：巧妙的利用栈来存储元素。

/** * // 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> {        /*借助栈来存放待遍历的元素，如果栈顶是整数则直接返回，否则继续把栈顶链表的元素放到栈里面*/    Stack<NestedInteger> stack = new Stack<NestedInteger>();//存放待遍历的元素(可能为一个链表也可能为一个整数)        public NestedIterator(List<NestedInteger> nestedList) {        if(nestedList == null){            return;        }        for(int i = nestedList.size()-1;i>=0;i--){            stack.push(nestedList.get(i));        }    }    @Override    public Integer next() {//返回栈顶元素即为本次遍历到的数字        return stack.pop().getInteger();    }    @Override    public boolean hasNext() {              while(!stack.isEmpty()){           NestedInteger top = stack.peek();           if(top.isInteger()){//说明现在的栈顶元素就是一个数字               return true;           }else{//如果栈顶元素是一个链表，则把其栈顶元素弹出，然后把该链表内的元素压栈               stack.pop();               for(int i= top.getList().size()-1;i>=0;i--){                   stack.push(top.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(); */