341.flatten-nested-list-iterator.java

/*
 * @lc app=leetcode id=341 lang=java
 *
 * [341] Flatten Nested List Iterator
 */

// @lc code=start
/**
 *  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> list;
    Iterator<Integer> it;
    public NestedIterator(List<NestedInteger> nestedList) {
        list = new ArrayList<>();
        for (NestedInteger ni : nestedList) {
            flatten(ni);
        }
        it = list.iterator();
    }

    @Override
    public Integer next() {
        return it.next();
    }

    @Override
    public boolean hasNext() {
        return it.hasNext();
    }

    private void flatten(NestedInteger ni) {
        if (ni == null) return;
        if (ni.isInteger()) list.add(ni.getInteger());
        else {
            List<NestedInteger> curList = ni.getList();
            for (NestedInteger n : curList) {
                flatten(n);
            }
        }
    }
}

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