215. Kth Largest Element in an Array.md

215. Kth Largest Element in an Array

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leetcode-cn Daily Challange on June 29th, 2020.

leetcode Daily Challange on January 16th, 2021.

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Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Example 1:

Input: [3,2,1,5,6,4] and k = 2
Output: 5

Example 2:

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4

Note:

• You may assume k is always valid, 1 ≤ k ≤ array's length.

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Solution

• mine
  • Java
    • Arrays.sort Runtime: 1 ms, faster than 97.99%, Memory Usage: 40 MB, less than 47.16% of Java online submissions

      //O(N*logN) O(N)space
      public int findKthLargest(int[] nums, int k) {
          Arrays.sort(nums);
          return nums[nums.length - k];
      }
      

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• leetcode solution

  • Qucik sort Runtime: 1 ms, faster than 97.99%, Memory Usage: 40.2 MB, less than 32.73% of Java online submissions

    //O(N)time O(logN)space
    Random random = new Random();
    
    public int findKthLargest(int[] nums, int k) {
        return quickSelect(nums, 0, nums.length - 1, nums.length - k);
    }
    
    public int quickSelect(int[] a, int l, int r, int index) {
        int q = randomPartition(a, l, r);
        if (q == index) {
            return a[q];
        } else {
            return q < index ? quickSelect(a, q + 1, r, index) : quickSelect(a, l, q - 1, index);
        }
    }
    
    public int randomPartition(int[] a, int l, int r) {
        int i = random.nextInt(r - l + 1) + l;
        swap(a, i, r);
        return partition(a, l, r);
    }
    
    public int partition(int[] a, int l, int r) {
        int x = a[r], i = l - 1;
        for (int j = l; j < r; ++j) {
            if (a[j] <= x) {
                swap(a, ++i, j);
            }
        }
        swap(a, i + 1, r);
        return i + 1;
    }
    
    public void swap(int[] a, int i, int j) {
        int temp = a[i];
        a[i] = a[j];
        a[j] = temp;
    }
    

  • Heap Sort Runtime: 1 ms, faster than 97.99%, Memory Usage: 40 MB, less than 44.18% of Java online submissions

    //O(N*logN)time O(logN)space
    public int findKthLargest(int[] nums, int k) {
        int heapSize = nums.length;
        buildMaxHeap(nums, heapSize);
        for (int i = nums.length - 1; i >= nums.length - k + 1; --i) {
            swap(nums, 0, i);
            --heapSize;
            maxHeapify(nums, 0, heapSize);
        }
        return nums[0];
    }
    
    public void buildMaxHeap(int[] a, int heapSize) {
        for (int i = heapSize / 2; i >= 0; --i) {
            maxHeapify(a, i, heapSize);
        }
    }
    
    public void maxHeapify(int[] a, int i, int heapSize) {
        int l = i * 2 + 1, r = i * 2 + 2, largest = i;
        if (l < heapSize && a[l] > a[largest]) {
            largest = l;
        }
        if (r < heapSize && a[r] > a[largest]) {
            largest = r;
        }
        if (largest != i) {
            swap(a, i, largest);
            maxHeapify(a, largest, heapSize);
        }
    }
    
    public void swap(int[] a, int i, int j) {
        int temp = a[i];
        a[i] = a[j];
        a[j] = temp;
    }
    

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