30-Day Leetcoding Challenge Day12

本题的难点在于每次要找到数组中前两个最大的值。提供三种解法：

1.每次对数组进行排序找到前两个最大的值

2.利用堆结构找到最大的两个值 PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder())

3.桶排序：虽然降低时间复杂度。但只适用于最大值 较小 时才可行。

JAVA

class Solution {
    public int lastStoneWeight(int[] stones) {
        List<Integer> _stones = new LinkedList<>();
        for(int stone : stones){
            _stones.add(stone);
        }
        Collections.sort(_stones);
        while(_stones.size() > 1){
            int x = _stones.remove(_stones.size()-2);
            int y = _stones.remove(_stones.size()-1);
            if(x != y){
                _stones.add(y-x);
                Collections.sort(_stones);
            }
        }
        return !_stones.isEmpty() ? _stones.get(0) : 0;
    }
}

class Solution {
    public int lastStoneWeight(int[] stones) {
        PriorityQueue<Integer> heap = new PriorityQueue<>(Comparator.reverseOrder());
        for(int stone: stones){
            heap.add(stone);
        }
        while(heap.size() > 1){
            int y = heap.remove();
            int x = heap.remove();
            if(x != y){
                heap.add(y-x);
            }
        }
        return !heap.isEmpty() ? heap.poll() : 0;
    }
}

class Solution {
    
    public int lastStoneWeight(int[] stones) {
        
        // Set up the bucket array.
        int maxWeight = stones[0];
        for (int stone: stones) {
            maxWeight = Math.max(maxWeight, stone);
        }
        int[] buckets = new int[maxWeight + 1];

        // Bucket sort.
        for (int weight : stones) {
            buckets[weight]++;
        }

        // Scan through the buckets.
        int biggestWeight = 0;
        int currentWeight = maxWeight;
        while (currentWeight > 0) {
            if (buckets[currentWeight] == 0) {
                currentWeight--;
            } else if (biggestWeight == 0) {
                buckets[currentWeight] %= 2;
                if (buckets[currentWeight] == 1) {
                    biggestWeight = currentWeight;
                }
                currentWeight--;
            } else {
                buckets[currentWeight]--;
                if (biggestWeight - currentWeight <= currentWeight) {
                    buckets[biggestWeight - currentWeight]++;
                    biggestWeight = 0;
                } else {
                    biggestWeight -= currentWeight;
                }
            }
        }
        return biggestWeight;
    }
}

Python3

class Solution:
    def lastStoneWeight(self, stones: List[int]) -> int:
        stones.sort(reverse = True)
        while len(stones) > 1:
            y = stones[0]
            x = stones[1]
            del stones[:2]
            if x != y:
                stones.append(y-x)
                stones.sort(reverse = True)
        return stones[0]

class Solution:
    def lastStoneWeight(self, stones: List[int]) -> int:
        for i in range(len(stones)):
            stones[i] *= -1
        heapq.heapify(stones)
        while len(stones) > 1:
            y = heapq.heappop(stones)
            x = heapq.heappop(stones)
            if x != y:
                heapq.heappush(stones,y-x)
        return -heapq.heappop(stones) if stones else 0

class Solution:
    def lastStoneWeight(self, stones: List[int]) -> int:
        
        # Set up the bucket array.
        max_weight = max(stones)
        buckets = [0] * (max_weight + 1)

        # Bucket sort.
        for weight in stones:
            buckets[weight] += 1

        # Scan through the weights.
        biggest_weight = 0 
        current_weight = max_weight
        while current_weight > 0:
            if buckets[current_weight] == 0:
                current_weight -= 1
            elif biggest_weight == 0:
                buckets[current_weight] %= 2
                if buckets[current_weight] == 1:
                    biggest_weight = current_weight
                current_weight -= 1
            else:
                buckets[current_weight] -= 1
                if biggest_weight - current_weight <= current_weight:
                    buckets[biggest_weight - current_weight] += 1
                    biggest_weight = 0
                else:
                    biggest_weight -= current_weight
        return biggest_weight