341. Flatten Nested List Iterator

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:
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].

-------------------------------
一种说法认为不应该把给出的list转换并存在本地，应该直接在上面遍历
阅读：Java的Iterator class和ListIterator class

没什么可说的

/**
 * // 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> {

    private int index;
    private List<Integer> list;
    public NestedIterator(List<NestedInteger> nestedList) {
        list = getList(nestedList);
        index = 0;
    }
    
    private List<Integer> getList(List<NestedInteger> nestedList) {
        List<Integer> l = new ArrayList<>();
        for (NestedInteger n : nestedList) {
            if (n.isInteger()) {
                l.add(n.getInteger());
            }else {
                l.addAll(getList(n.getList()));
            }
        }
        
        return l;
    }

    @Override
    public Integer next() {
        int save = list.get(index);
        index++;
        return save;
    }

    @Override
    public boolean hasNext() {
        return index < list.size();
    }
}

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

Solution #2, 用stack
ToDo: https://leetcode.com/problems/flatten-nested-list-iterator/discuss/80147/Simple-Java-solution-using-a-stack-with-explanation