12 Apr 2020 | Leetcode 30-Day Challenge: Day 12

Last Stone Weight

Difficulty: Easy

Problem statement:

We have a collection of stones, each stone has a positive integer weight.

Each turn, we choose the two heaviest stones and smash them together. Suppose the stones have weights x and y with x <= y. The result of this smash is:

If x == y, both stones are totally destroyed;
If x != y, the stone of weight x is totally destroyed, and the stone of weight y has new weight y-x.
At the end, there is at most 1 stone left. Return the weight of this stone (or 0 if there are no stones left.)

Example 1:

Input: [2,7,4,1,8,1]
Output: 1
We combine 7 and 8 to get 1 so the array converts to [2,4,1,1,1] then,
we combine 2 and 4 to get 2 so the array converts to [2,1,1,1] then,
we combine 2 and 1 to get 1 so the array converts to [1,1,1] then,
we combine 1 and 1 to get 0 so the array converts to [1] then that's the value of last stone.


1 <= stones.length <= 30
1 <= stones[i] <= 1000


Happy rainy Omaha Sunday! Today’s challenge wasn’t too bad. I read through the problem a few times and wrote some code and happen to solve it within a few runs.

The time complexity on this is dependent upon the call to sort.Ints([]int). Looking at the source code, we can see that Go implements quicksort for this. We can reference Big-O Cheat Sheet (or google really) to see that we’re on average getting Θ(n log(n)) for this. I don’t claim to be a master of asymptotic analysis, but I believe having this quicksort inside my while loop promotes this solution to O(n²) (please correct me if I’m mistaken).

That being said, our solution’s runtime ended up being 0 milliseconds. Space complexity would be O(n)

func lastStoneWeight(stones []int) int {
    x, y, err := getHeaviestStones(stones)
    for err == nil {
        stones = smashStones(x, y, stones)
        x, y, err = getHeaviestStones(stones)  
    if len(stones) > 0 {
      return stones[0]
    return 0

func smashStones(x, y int, stones []int) []int {
    if x == y {
        stones = stones[:len(stones) - 2]
    } else {
        stoneY := y - x
        stones = stones[:len(stones) - 1] 
        stones[len(stones) - 1] = stoneY
    return stones

func getHeaviestStones(stones []int) (int, int, error) {
    if len(stones) < 2 {
        return -1, -1, errors.New("Array size must be at least two!")
    return stones[len(stones) - 2], stones[len(stones) - 1], nil

Solution Details

Runtime: 0 ms Memory Usage: 2 MB