341. Flatten Nested List Iterator

341. 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> {
    
    private Stack<NestedInteger> stack=new Stack<NestedInteger>();

    public NestedIterator(List<NestedInteger> nestedList) {
        for(int i=nestedList.size()-1;i>=0;i--){//把nestedList的值全部放到栈中，从nestedList的尾部开始压栈
            stack.push(nestedList.get(i));
        }
    }

    @Override
    public Integer next() {
        /*
        *刚开始写成
        *stack.pop();
        *return stack.peek().getInteger();
        *发现返回null,因为还没取值就把栈顶元素pop了
        */
        Integer i=stack.peek().getInteger();
        stack.pop();
        return i;
    }

    @Override
    public boolean hasNext() {
        /*
        *刚开始把while放在了NestedIterator方法中，发现只执行了一次的list重新出栈和压栈，后面的list没有进入while操作
        *因为NestedIterator()方法只执行了一次，是构造函数
        */
         while(!stack.isEmpty() && !stack.peek().isInteger()){
            List<NestedInteger> list1=stack.peek().getList();
            stack.pop();
            for(int i=list1.size()-1;i>=0;i--){
                stack.push(list1.get(i));
            }
        }
        if(!stack.isEmpty()){
            return true;
        }
        return false;
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */