[LeetCode] 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].

解题思路

首先递归把integer元素集中到一个list中，然后再借助list的迭代器进行遍历。

实现代码

// Runtime: 6 ms/** * // 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 List<Integer> nums = new ArrayList<>();    private Iterator<Integer> iterator = null;    public NestedIterator(List<NestedInteger> nestedList) {        for (NestedInteger nestedInteger : nestedList) {            addNestedInteger(nestedInteger);        }        iterator = nums.iterator();    }    @Override    public Integer next() {        return iterator.next();    }    @Override    public boolean hasNext() {        return iterator.hasNext();    }    private void addNestedInteger(NestedInteger nestedInteger) {        if (nestedInteger.isInteger()) {            nums.add(nestedInteger.getInteger());        } else {            List<NestedInteger> nestedList = nestedInteger.getList();            for (NestedInteger nested : nestedList) {                addNestedInteger(nested);            }        }    }}/** * Your NestedIterator object will be instantiated and called as such: * NestedIterator i = new NestedIterator(nestedList); * while (i.hasNext()) v[f()] = i.next(); */