Added Kth largest

Author: FreeTymeKiyan

Other/KthLargest.java

@@ -0,0 +1,77 @@
+import java.util.*;
+
+/**
+ * k largest(or smallest) elements in an array
+ * 
+ * Tags: Array, Sort, Heap
+ */
+class KthLargest {
+    public static void main(String[] args) {
+        KthLargest K = new KthLargest();
+        // int[] A = { 1, 3, 5, 2, 4, 6 };
+        int[] A = {1, 23, 12, 9, 30, 2, 50};
+        for (int k = 1; k <= A.length; k++) {
+            // System.out.println(K.findKthLargest(A, k));
+            System.out.println(K.findKthLargestB(A, k));
+        }
+    }
+    
+    /**
+     * Priority Queue
+     * O(n) + k * O(logn)
+     */
+    public int findKthLargest(int[] A, int k) {
+        if (k <= 0 || k > A.length) return -1;
+        Queue<Integer> q = new PriorityQueue<Integer>(A.length, Collections.reverseOrder());
+        for (int n : A) q.add(n);
+        int res = 0;
+        for (int i = 0; i < k; i++) res = q.poll();
+        return res;
+    }
+    
+    /**
+     * QuickSelect
+     * Use partition algorithm in Quick Sort
+     */
+    public int findKthLargestB(int[] A, int k) {
+        if (k <= 0 || k > A.length) return -1;
+        
+        int l = 0; // initialize
+        int r = A.length - 1;
+        int index;
+        
+        while (l < r) {
+            index = partition(A, l, r);
+            if (index > k - 1) {
+                r = index - 1;
+            } else if (index < k - 1) {
+                l = index + 1;
+            } else {
+                return A[index];
+            } 
+        }
+        return A[l];
+    }
+    
+    /**
+     * Choose mid value as pivot
+     * Move two pointers
+     * Swap and move on
+     * Return left pointer
+     */
+    private static int partition(int[] a, int left, int right) {
+        int pivot = a[left + (right - left) / 2];
+        while(left <= right) {
+            while(a[left] > pivot) left++;
+            while(a[right] < pivot) right--;
+            if (left <= right) {
+                int temp = a[left];
+                a[left] = a[right];
+                a[right] = temp;
+                left++;
+                right--;
+            }
+        }
+        return left;
+    }
+}