FlattenNestedListIterator.java

/*
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]. 
*/


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

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

    @Override
    public boolean hasNext() {
        while(!st.isEmpty()){
            NestedInteger cur = st.peek();
            if(cur.isInteger()){
                return true;
            }else{
                st.pop();
                List<NestedInteger> q = cur.getList();
                int s = q.size();
                for(int i = s-1; i>=0; i--){
                    st.push(q.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();
 */	
	public static void main(String[] args) {
		// testing ignored...
		return;
	}	
}