add QuickSort and TopK algorithm

Main.java

@@ -0,0 +1,10 @@
+public class Main{
+    public static void main(String[] args){
+	QuickSort lc = new QuickSort();
+        int[] nums = {4, 2, 3, 1,5, 7,6,6};
+	lc.quickSort(nums, 0, nums.length - 1);
+	for(int i = 0; i < nums.length; ++i){
+            System.out.println(nums[i]);
+        }
+    }
+}

QuickSort/QuickSort.java

@@ -0,0 +1,26 @@
+public class QuickSort{
+    public void quickSort(int[] nums, int start, int end){
+	if(start >= end) return;
+	int pos = partition(nums, start, end);
+        quickSort(nums, start, pos - 1);
+        quickSort(nums, pos + 1, end);
+    }
+
+    public int partition(int[] nums, int start, int end){
+        int pivot = nums[end];
+        int pos = start - 1, tmp = 0;
+        for(int i = start; i < end; ++i){
+            if(nums[i] <= pivot){
+     		++pos;
+		tmp = nums[pos];    // swap
+ 		nums[pos] = nums[i];
+		nums[i] = tmp;
+            }
+        }
+	// swap 
+	nums[end] = nums[pos + 1];
+	nums[pos + 1] = pivot;
+
+	return pos + 1;
+    }
+}

QuickSort/TopK.java

@@ -0,0 +1,34 @@
+public class TopK{
+    public void getTopK(int nums[], int k){
+        int start = 0, end = nums.length - 1;
+        int pos = partition(nums, start, end);
+	while(pos != k - 1){
+	    if(pos > k - 1){
+		end = pos - 1;
+            }else{
+		start = pos + 1;
+            }
+	    pos = partition(nums, start, end);
+        }
+	for(int i = 0; i < k; ++i)
+	    System.out.println(nums[i]);
+    }
+
+    public int partition(int[] nums, int start, int end){
+        int pivot = nums[end];
+	int pos = start - 1, tmp = 0;
+	for(int i = start; i < end; ++i){
+            if(nums[i] <= pivot){
+	        ++pos;
+		//swap
+		tmp = nums[pos];
+		nums[pos] = nums[i];
+		nums[i] = tmp;
+            }
+        }
+	// swap
+	nums[end] = nums[pos + 1];
+	nums[pos + 1] = pivot;
+	return pos + 1;
+    }
+}