Question

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].

Thinking:

  • Method 1:
    • 将链表添加进入的时候需要用到递归。
    • 使用队列来装载数据。
/**
 * // 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 LinkedList<Integer> list;
    public NestedIterator(List<NestedInteger> nestedList) {
        this.list = new LinkedList<>();
        addList(nestedList);
    }
    private void addList(List<NestedInteger> list){
        for(NestedInteger num : list){
            if(num.isInteger()){
                this.list.add(num.getInteger());
            }else
                addList(num.getList());
        }
    }

    @Override
    public Integer next() {
        return this.list.poll();
    }

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

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