[随缘一题]平面列表

给定 [1,2,[1,2]]，返回 [1,2,1,2]。

给定 [4,[3,[2,[1]]]]，返回 [4,3,2,1]。

// 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 List<Integer> flatten1(List<NestedInteger> nestedList) {
  List<Integer> result = new ArrayList<>();
  for (int i = 0; i < nestedList.size(); i++) {
    //是整数则添加到结果集
    if (nestedList.get(i).isInteger()) {
      result.add(nestedList.get(i).getInteger());
      continue;
    }
    //是列表则递归调用将结果全添加到结果集中
    result.addAll(flatten1(nestedList.get(i).getList()));
  }
  return result;
}

public List<Integer> flatten2(List<NestedInteger> nestedList) {
  List<Integer> result = new LinkedList<>();
  Stack<NestedInteger> stack = new Stack<>();
  //初始全部入栈
  nestedList.forEach((stack::push));
  while (!stack.isEmpty()) {
    NestedInteger current = stack.pop();
    if (current == null) {
      continue;
    }
    //当前为整数则添加到结果集
    if (current.isInteger()) {
      ((LinkedList<Integer>) result).addFirst(current.getInteger());
    } else {
      //否则遍历列表将元素全部入栈
      current.getList().forEach(stack::push);
    }
  }
  return result;
}