smallSum traning

src/leo/class02/SmallSum.java

@@ -272,6 +272,48 @@ class SmallSum5{
     }
 }
 
+class SmallSum6{
+    public static int smallSum(int[] arr) {
+        if (arr.length < 2 || arr == null) {
+            return 0;
+        }
+
+        return process(arr, 0, arr.length - 1);
+    }
+
+    private static int process(int[] arr, int l, int r) {
+        if (l == r) {
+            return 0;
+        }
+        int m = l + ((r - l) >> 1);
+        return process(arr, l, m) + process(arr, m + 1, r) + merge(arr, l, m, r);
+
+    }
+
+    private static int merge(int[] arr, int l, int m, int r) {
+
+        int[] help = new int[r - l + 1];
+        int res = 0;
+        int p1 = l;
+        int p2 = m + 1;
+        int i = 0;
+        while (p1 <= m && p2 <= r) {
+            res += arr[p1] < arr[p2] ? (r - p2 + 1) * arr[p1] : 0;
+            help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
+        }
+        while (p1 <= m) {
+            help[i++] = arr[p1++];
+        }
+        while (p2 <= r) {
+            help[i++] = arr[p2++];
+        }
+        for (i = 0; i < help.length; i++) {
+            arr[l + i] = help[i];
+        }
+        return res;
+    }
+}
+
 class TestSmallSum{
     public static int smallSum(int[] arr) {
         if (arr.length < 2 || arr == null) {
@@ -315,7 +357,7 @@ class TestMain {
         for (int i = 0; i < testTime; i++) {
             int[] arr = ArrayUtil.randomArray(maxSize, range);
             int[] copyArray = ArrayUtil.copyArray(arr);
-            int sum = SmallSum5.smallSum(arr);
+            int sum = SmallSum6.smallSum(arr);
             int testSum = TestSmallSum.smallSum(copyArray);
             if (testSum != sum) {
                 System.out.println("sum :" + sum + ", testSum : " + testSum);