341. Flatten Nested List Iterator

Question

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

initialize iterator with nestedList
res = []
while iterator.hasNext()
append iterator.next() to the end of res
return res

If res matches the expected flattened list, then your code will be judged as correct.

Solution

将所有的NestedInteger展开后加入全局变量队列。

辅助方法buildQueue():
递归，如果当前元素是列表，则向下递归列表内的所有元素。
如果当前元素是单个整数，则将其加入队列。

next():
返回并挤出队列中的下一个元素，返回其整数。

hasNext():
返回队列是否为空。

Code

/**
 * // 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 empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    Queue<NestedInteger> q;
    public NestedIterator(List<NestedInteger> nestedList) {
        q = new LinkedList<>();
        for(NestedInteger item : nestedList){
            buildQueue(item);
        }
    }
    
    private void buildQueue(NestedInteger ni){
        if(!ni.isInteger()){
            for(NestedInteger item : ni.getList()){
                buildQueue(item);
            }
        }
        else{
            q.offer(ni);
        }
    }

    @Override
    public Integer next() {
        return q.poll().getInteger();
    }

    @Override
    public boolean hasNext() {
        return !q.isEmpty();
    }
}

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