341. 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.

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].

/**
 * // 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> {
    Stack<NestedInteger> stack = new Stack();
    public NestedIterator(List<NestedInteger> nestedList) {
        for(int i = nestedList.size() - 1; i >=0; i--){
            stack.push(nestedList.get(i));
        }
    }

    @Override
    public Integer next() {
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!stack.isEmpty()){
            NestedInteger cur = stack.peek();
            if(cur.getInteger() != null) return true;
            //不是Integer的时候才开始flatten，毕竟只是判断hasNext
            stack.pop();
            for(int i = cur.getList().size() - 1; i >= 0; i--){
                stack.push(cur.getList().get(i));
            }
        }
         return false;
    }
}

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

In the constructor, we push all the nestedList into the stack from back to front, so when we pop the stack, it returns the very first element. Second, in the hasNext() function,

we peek the first element in stack currently, and if it is an Integer, we will return true and pop the element. If it is a list, we will further flatten it.

This is iterative version of flatting the nested list. Again, we need to iterate from the back to front of the list.

https://leetcode.com/problems/flatten-nested-list-iterator/discuss/80147/Simple-Java-solution-using-a-stack-with-explanation