leetcode341. Flatten Nested List Iterator

題目要求

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].

假設有一個嵌套形式的數組，要求按照順序遍歷數組中的元素。

思路和代碼

首先能夠想到經過深度優先遞歸的方式將嵌套形式的數組展開爲一個無嵌套的列表。那麼有沒有方法實現不將元素取出而是直接迭代的方式獲取下一個元素呢？這裏採用了嵌套迭代器的方式進行遍歷

public class FlattenNestedListIterator_341 implements Iterator<Integer>{

        private List<NestedInteger> nestedList;
        private Iterator<Integer> iterator;
        private int next;
        private int index;
        public FlattenNestedListIterator_341(List<NestedInteger> nestedList){
            this.nestedList = nestedList;
        }
        
         @Override
        public Integer next() {
             return next;
        }

        @Override
        public boolean hasNext() {
            if(iterator != null && iterator.hasNext()){
                this.next = iterator.next();
                return true;
            }
            
            if(index>=nestedList.size()){
                return false;
            }
            
            NestedInteger n = nestedList.get(index++);
            if(n.isInteger()){
                next = n.getInteger();
            }else{
                iterator = new FlattenNestedListIterator_341(n.getList());
                if(iterator.hasNext()){
                    next = iterator.next();
                }else{
                    return hasNext();
                }
            }
            return true;
        }
}

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