1 public class FindEqualSum {
 2
 3     public static void main(String[] args) {
 4         int[] arr = { 2, 1, 21, 16, 32, 35, 45, 31 };
 5         findEqualSum(arr, 33);
 6     }
 7
 8     private static void findEqualSum(int[] arr, int num) {
 9         int startIndex = 0;
10         int endIndex = arr.length - 1;
11
12         // 先进行归并排序，然后再找两数之和
13         divideSort(arr, startIndex, endIndex);
14         findInSorted(arr, startIndex, endIndex, num);
15     }
16
17     private static void divideSort(int[] arr, int startIndex, int endIndex) {
18         if (startIndex >= endIndex) {
19             return;
20         }
21         int midIndex = (startIndex + endIndex) / 2;
22         divideSort(arr, startIndex, midIndex);
23         divideSort(arr, midIndex + 1, endIndex);
24         merge(arr, startIndex, midIndex, endIndex);
25     }
26
27     private static void findInSorted(int[] arr, int startIndex, int endIndex,
28             int num) {
29         int i = startIndex;
30         int j = endIndex;
31         while (i < j) {
32             if (arr[i] + arr[j] > num) { // 如果两数之和大于要找的数说明有一个数过大，这时候需要前移后面较大的数
33                 j--;
34             } else if (arr[i] + arr[j] < num) { // 如果两数之和小于要找的数，说明有一个数要小，这时应该后移前面较小的数
35                 i++;
36             } else { // 相等这输出找到的信息，这时候如果需要找到所有需要记住仍要前移后一个数或者后移前一个数，防止死循环。
37                 System.out.println(arr[i] + " + " + arr[j] + " = "
38                         + (arr[i] + arr[j]));
39                 j--;
40             }
41         }
42     }
43
44     private static void merge(int[] arr, int startIndex, int midIndex,
45             int endIndex) {
46         int k = 0;
47         int i = startIndex;
48         int j = midIndex + 1;
49         int[] newArr = new int[endIndex - startIndex + 1];
50         while (i <= midIndex && j <= endIndex) {
51             if (arr[i] > arr[j]) {
52                 newArr[k++] = arr[j++];
53             } else {
54                 newArr[k++] = arr[i++];
55             }
56         }
57
58         if (i <= midIndex)
59         {
60             System.arraycopy(arr, i, newArr, k, midIndex - i + 1);
61         }
62         if (j <= endIndex)
63         {
64             System.arraycopy(arr, j, newArr, k, endIndex - j + 1);
65         }
66         System.arraycopy(newArr, 0, arr, startIndex, endIndex - startIndex + 1);
67     }
68
69 }

 1 private static void merge(int[] arr, int startIndex, int midIndex,
 2             int endIndex) {
 3         int k = 0;
 4         int i = startIndex;
 5         int j = midIndex + 1;
 6         int[] newArr = new int[endIndex - startIndex + 1];
 7         while (i <= midIndex && j <= endIndex) {
 8             if (arr[i] > arr[j]) {
 9                 count++;    // 这里用来记录逆序对的个数
10                 newArr[k++] = arr[j++];
11             } else {
12                 newArr[k++] = arr[i++];
13             }
14         }
15
16         if (i <= midIndex)
17         {
18             System.arraycopy(arr, i, newArr, k, midIndex - i + 1);
19         }
20         if (j <= endIndex)
21         {
22             System.arraycopy(arr, j, newArr, k, endIndex - j + 1);
23         }
24         System.arraycopy(newArr, 0, arr, startIndex, endIndex - startIndex + 1);
25     }