Leetcode 341 - Flatten nested list iterator

Note:

  • Have to use getInteger and getList, otherwise it won’t work.
  • Use stack to store nestedList.
  • Because we call hasNext before next every time, so we can flatten the top of the stack at that time.
  • Iterate backwards on top element, and stop when this.stack.top.isInteger == true.
  • Be careful about [[]]. We need an exception check before iterating.

Question:

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

  • NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
  • int next() Returns the next integer in the nested list.
  • boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

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initialize iterator with nestedList
res = []
while iterator.hasNext()
append iterator.next() to the end of res
return res

If res matches the expected flattened list, then your code will be judged as correct.

Example:

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Input: nestedList = [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Code:

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/**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* function NestedInteger() {
*
* Return true if this NestedInteger holds a single integer, rather than a nested list.
* @return {boolean}
* this.isInteger = function() {
* ...
* };
*
* Return the single integer that this NestedInteger holds, if it holds a single integer
* Return null if this NestedInteger holds a nested list
* @return {integer}
* this.getInteger = function() {
* ...
* };
*
* Return the nested list that this NestedInteger holds, if it holds a nested list
* Return null if this NestedInteger holds a single integer
* @return {NestedInteger[]}
* this.getList = function() {
* ...
* };
* };
*/
/**
* @constructor
* @param {NestedInteger[]} nestedList
*/
var NestedIterator = function (nestedList) {
this.stack = [];
for (let i = nestedList.length - 1; i >= 0; i--) {
this.stack.push(nestedList[i]);
}
};


/**
* @this NestedIterator
* @returns {boolean}
*/
NestedIterator.prototype.hasNext = function () {
if (len === 0) return false;
while (this.stack.length > 0) {
const top = this.stack[this.stack.length - 1];
// Edge case check
if (!top) {
this.stack.pop();
continue;
}
if (top.isInteger()) {
return true;
} else {
const cur = top.getList();
this.stack.pop();
for (let i = cur.length - 1; i >= 0; i--) {
this.stack.push(cur[i]);
}
}
}
};

/**
* @this NestedIterator
* @returns {integer}
*/
NestedIterator.prototype.next = function () {
const top = this.stack.pop();
return top.getInteger();
};

/**
* Your NestedIterator will be called like this:
* var i = new NestedIterator(nestedList), a = [];
* while (i.hasNext()) a.push(i.next());
*/