341 flatten nested list iterator

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

卡了半天，其实你不能边dfs边返回值，你只能一开始就一口气展开

/**
 * // 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 empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    private List<NestedInteger> list;
    private List<Integer> ans;
    private NestedInteger cur;
    private int index = 0;
    public NestedIterator(List<NestedInteger> nestedList) {
        list = nestedList;
        ans = new ArrayList<>();
        for (NestedInteger node : list) {
            span(node);
        }
    }
    private void span(NestedInteger node) {
        if (node.isInteger()) {
            ans.add(node.getInteger());
            return;
        }
        List<NestedInteger> children = node.getList();
        for (NestedInteger child : children) {
            span(child);
        }
    }

    @Override
    public Integer next() {
        return ans.get(index++);
    }

    @Override
    public boolean hasNext() {
        if (ans == null || ans.isEmpty() || index == ans.size()) return false;
        return true;
    }
}

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