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341. 扁平化嵌套列表迭代器

给你一个嵌套的整型列表。请你设计一个迭代器，使其能够遍历这个整型列表中的所有整数。

列表中的每一项或者为一个整数，或者是另一个列表。其中列表的元素也可能是整数或是其他列表。

链接

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *     If value is provided, then it holds a single integer
 *     Otherwise it holds an empty nested list
 *     constructor(value?: number) {
 *         ...
 *     };
 *
 *     Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     isInteger(): boolean {
 *         ...
 *     };
 *
 *     Return the single integer that this NestedInteger holds, if it holds a single integer
 *     Return null if this NestedInteger holds a nested list
 *     getInteger(): number | null {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a single integer equal to value.
 *     setInteger(value: number) {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a nested list and adds a nested integer elem to it.
 *     add(elem: NestedInteger) {
 *         ...
 *     };
 *
 *     Return the nested list that this NestedInteger holds,
 *     or an empty list if this NestedInteger holds a single integer
 *     getList(): NestedInteger[] {
 *         ...
 *     };
 * };
 */

class NestedIterator {
  stack: NestedInteger[]
  constructor(nestedList: NestedInteger[]) {
    this.stack = nestedList;
  }

  hasNext(): boolean {
    if (this.stack.length > 0) {
      if (this.stack[0].isInteger()) {
        return true
      } else {
        const cur = this.stack[0].getList()
        this.stack.shift()
        this.stack.unshift(...cur)
        return this.hasNext()
      }
    } else {
      return false
    }
  }
  
  next(): number {
    return this.stack.shift().getInteger()
  }
}

/**
 * Your ParkingSystem object will be instantiated and called as such:
 * var obj = new NestedIterator(nestedList)
 * var a: number[] = []
 * while (obj.hasNext()) a.push(obj.next());
 */