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.

Notice You don't need to implement the remove method.

Example

• 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].

• 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].

题目中的NestedInteger应该是实际当中General Grouping Object (or Container)的特例。看到这种数据结构的第一反应就是用stack。注意倒叙插入即可。

/**
 * // 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();
 * }
 */
import java.util.Iterator;

public class NestedIterator implements Iterator<Integer> {
    
    Stack<NestedInteger> stack;
    public NestedIterator(List<NestedInteger> nestedList) {
        // Initialize your data structure here.
        stack = new Stack<NestedInteger>();
        if(nestedList!=null){
            for(int i=nestedList.size()-1;i>=0;i--){
                stack.push(nestedList.get(i));
            }
        }
    }

    // @return {int} the next element in the iteration
    @Override
    public Integer next() {
        // Write your code here
        return stack.pop().getInteger();
    }

    // @return {boolean} true if the iteration has more element or false
    @Override
    public boolean hasNext() {
        // Write your code here
        while(!stack.empty()){
            NestedInteger cur = stack.peek();
            if(cur.isInteger()) return true;
            stack.pop();
            List<NestedInteger> list = cur.getList();
            if(list!=null){
                for(int i=list.size()-1;i>=0;i--){
                    stack.push(list.get(i));
                }
            }
        }
        return false;
    }

    @Override
    public void remove() {}
}

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