251. Flatten 2D Vector 平铺矩阵

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:

Input: [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false, 
             the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

Input: [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false, 
             the order of elements returned by next should be: [1,4,6].


思路：for each的循环!判断i是不是integer,如果不是，就继续flatten它

注意要初始化

flattenedList = new LinkedList<Integer>();

public class NestedIterator implements Iterator<Integer> {
    LinkedList<Integer> flattenedList;
    Iterator<Integer> iterator;

    public NestedIterator(List<NestedInteger> nestedList) {
        //注意要初始化
        flattenedList = new LinkedList<Integer>();
        flatten(nestedList);
        iterator = flattenedList.iterator();
    }
    
    public void flatten(List<NestedInteger> nestedList) {
        for (NestedInteger ni : nestedList) {
            if (ni.isInteger()) {
                flattenedList.add(ni.getInteger());
            }else {
                flatten(ni.getList());
            }
        }
    }

    @Override
    public Integer next() {
        return iterator.next();
    }

    @Override
    public boolean hasNext() {
        return iterator.hasNext();
    }
}

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