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]
思路: 用一个List来盛放flatten之后的元素,同时用一个index来取当前flatten list的元素。

/**
 * // 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> {
    List<Integer> flatList;
    int index;

    public NestedIterator(List<NestedInteger> nestedList) {
        flatList = new ArrayList<Integer>();
        index = 0;
        dumpToList(nestedList, flatList);
    }

    public void dumpToList(List<NestedInteger> nestedList, List<Integer> flatList) {
        for (NestedInteger ni : nestedList) {
            if (ni.isInteger()) {
                flatList.add(ni.getInteger());
            } else {
                dumpToList(ni.getList(), flatList);
            }
        }
    }


    @Override
    public Integer next() {
        if (hasNext()) {
            Integer result = flatList.get(index);
            index++;
            return result;
        } 
        return null;
    }

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

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