Poison

215. Kth Largest Element in an Array

发表于
Priority Queue
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class Solution {
    public int findKthLargest(int[] nums, int k) {
        Queue<Integer> minHeap = new PriorityQueue<>(); // 小顶堆，小于堆顶的都被过滤了，剩下的就是 topK
        for (int i = 0; i < k; i++) {
            minHeap.offer(nums[i]);
        }

        for (int i = k; i < nums.length; i++) {
            if (nums[i] > minHeap.peek()) { // if 判断不要亦可，具有该判断时性能更优，即只将比堆顶数值更大的数值入堆
                minHeap.offer(nums[i]);
                minHeap.poll();
            }
        }

        return minHeap.peek();
    }
}
QuickSort
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class Solution {
    public int findKthLargest(int[] nums, int k) {
        int targetIndex = k - 1;

        int left = 0, right = nums.length - 1;
        while (true) {
            int pivotIndex = partition(nums, left, right);
            if (pivotIndex < targetIndex) {
                left = pivotIndex + 1;
            } else if (pivotIndex > targetIndex) {
                right = pivotIndex - 1;
            } else {
                return nums[pivotIndex];
            }
        }
    }

    private int partition(int[] nums, int left, int right) {
        // 随机选择一个元素交换至最左侧，性能优化使用，亦可不交换
        int randomIndex = left + ThreadLocalRandom.current().nextInt(right - left + 1);
        swap(nums, left, randomIndex);

        int pivotValue = nums[left];
        int endIndex = left; // 指向左边界（含）元素
        for (int i = left + 1; i <= right; i++) {
            if (nums[i] >= pivotValue) {
                swap(nums, i, ++endIndex);
            }
        }
        // nums[left + 1, ... endIndex] >= nums[left]
        swap(nums, left, endIndex);
        return endIndex;
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}
QuickSort + Two Pointers
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class Solution {
    public int findKthLargest(int[] nums, int k) {
        int targetIndex = k - 1;

        // sort to descending

        int left = 0, right = nums.length - 1;
        while (true) {
            int pivotIndex = partition(nums, left, right);
            if (pivotIndex < targetIndex) {
                left = pivotIndex + 1;
            } else if (pivotIndex > targetIndex) {
                right = pivotIndex - 1;
            } else {
                return nums[pivotIndex];
            }
        }
    }

    private int partition(int[] nums, int left, int right) {
        int randomIndex = left + ThreadLocalRandom.current().nextInt(right - left + 1);
        swap(nums, left, randomIndex);

        int pivotValue = nums[left];
        int i = left, j = right;
        while (i < j) {
            while (i < j && nums[j] <= pivotValue) {
                j--;
            }
            while (i < j && nums[i] >= pivotValue) {
                i++;
            }
            swap(nums, i, j);
        }

        swap(nums, left, i);
        return i;
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}
Reference

215. Kth Largest Element in an Array