341.Flatten Nested List Iterator

1-Stack

https://leetcode.com/problems/flatten-nested-list-iterator/discuss/80147/

Very Useful Comment

a small comment is that your solution works only if we always call hasNext() before calling next(). certainly meet requirement of this problem but not generic for iterator. a small modification can make it more generic, if needed.

/**
 * // 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> {
    Deque<NestedInteger>  stack = new ArrayDeque<>();

    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new ArrayDeque<>();
        flattenList(nestedList);
    }

    @Override
    public Integer next() {
        if(hasNext())
            return stack.pop().getInteger();
        else 
            return null;
    }

    @Override
    public boolean hasNext() {
        while(!stack.isEmpty()){
            if(stack.peek().isInteger())
                return true;
            flattenList(stack.pop().getList());
        }
        return false;
    }

    private void flattenList(List<NestedInteger> list){
        for(int i = list.size() - 1; i >= 0; i--){
            stack.push(list.get(i));
        }
    }
}

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