341. Flatten Nested List Iterator展开多层数组

［抄题］：

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].

[暴力解法]：

时间分析：

空间分析：

[优化后]：

时间分析：

空间分析：

［奇葩输出条件］：

［奇葩corner case］：

［思维问题］：

不知道.next 和 .hasnext有啥区别：取出来、只是看看有没有

［一句话思路］：

只有stack才能一次取出来一层，getlist getinteger

［输入量］：空：正常情况：特大：特小：程序里处理到的特殊情况：异常情况（不合法不合理的输入）：

［画图］：

［一刷］：

.next建立在.hasnext的基础之上，所以hasnext需要放在while循环中，做完为止
curr如果是个list，就必须用专有方法
```
curr.getList()
```
先取出，再做后续操作

［二刷］：

［三刷］：

［四刷］：

［五刷］：

[五分钟肉眼debug的结果]：

［总结］：

一次放一层

［复杂度］：Time complexity: O(n) Space complexity: O(n)

［英文数据结构或算法，为什么不用别的数据结构或算法］：

list 对应的方法是.size() .get

只有stack才能一次取出来一层，数组不能直接取出来一层。所以用stack。

stack有

.getInteger()

.getList()

方法

［算法思想：递归/分治/贪心］：

［关键模板化代码］：

public NestedIterator(List<NestedInteger> nestedList) {

    //put into stack from back

        for (int i = nestedList.size() - 1; i >= 0; i--) stack.push(nestedList.get(i));

    }

［其他解法］：

［Follow Up］：

［LC给出的题目变变变］：

[代码风格] ：

/**

 * // 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> {

    //ini:stack

    Stack<NestedInteger> stack = new Stack<>();

    public NestedIterator(List<NestedInteger> nestedList) {

    //put into stack from back

        for (int i = nestedList.size() - 1; i >= 0; i--) stack.push(nestedList.get(i));

    }

    @Override

    public Integer next() {

    //pop

        return stack.pop().getInteger();

    }

    @Override

    public boolean hasNext() {

        while (!stack.isEmpty()) {

        //isInteger or put into stack from back

        NestedInteger curr = stack.peek();

        if (curr.isInteger()) return true;

        stack.pop();

        for (int i = curr.getList().size() - 1; i >= 0; i--) {

            stack.push(curr.getList().get(i));

        }

        }

        return false;

    }

}

/**

 * Your NestedIterator object will be instantiated and called as such:

 * NestedIterator i = new NestedIterator(nestedList);

 * while (i.hasNext()) v[f()] = i.next();

 */

巴特西

341. Flatten Nested List Iterator展开多层数组

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