Leetcode/G家F家 -- 341. Flatten Nested List Iterator (Design + Recur)

341. Flatten Nested List Iterator(Design + Recur)
medium
https://leetcode.com/problems/flatten-nested-list-iterator/
相关题： 同样用recursion traverse list 来计算
http://rainykat.blogspot.com/2017/01/leetcodelinkedin-339364-nested-list.html 

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].

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思路： traverse the nested list using recursion and store element to arraylist.

/**
 * // 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 = new ArrayList<>();
    int pos = 0;//current position
    public NestedIterator(List<NestedInteger> nestedList) {
        //use arrayList to store nestedList
        traverse(nestedList);
    }
    public void traverse(List<NestedInteger> nestedList){
        if(nestedList == null) return;
        for(NestedInteger e: nestedList){
            if(e.isInteger()){
                list.add(e.getInteger());
            }else{
                traverse(e.getList());//do recursion when meeting list element
            }
        }
    }
    @Override
    public Integer next() {
        return list.get(pos++);
    }

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

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