Leetcode: 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:
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].

非常精巧地使用stack。push all the nestedList into the stack from back to front，so when we pop the stack, it returns the very first element

执行hasNext()的时候，如果peek()是integer, return true；否则是一个List<NestedInteger>, 则flatten这个list，同样的，把list里的NestedInteger倒序push入栈

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

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

    @Override
    public boolean hasNext() {
        while (!st.isEmpty()) {
            if (st.peek().isInteger()) return true;
            List<NestedInteger> cur = st.pop().getList();
            for (int i=cur.size()-1; i>=0; i--) {
                st.push(cur.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();
 */