【算法习题】数组中任意2个（3个）数的和为sum的组合

题1、给定一个int数组，一个数sum，求数组中和为sum的任意2个数的组合

 @Test
 public void test_find2() {
     int[] arr = { -1, 0, 2, 3, 4, 7, 8, 9, 10 };
     int sum = 9;
     Arrays.sort(arr);
     List<TwoTuple<Integer, Integer>> result = new ArrayList<>();

     int i = 0;
     int j = arr.length - 1;
     while (i < j) {
         if (arr[i] + arr[j] == sum) {
             result.add(new TwoTuple<Integer, Integer>(arr[i], arr[j]));
             i++;
             j--;
         } else if (arr[i] + arr[j] < sum) {
             i++;
         } else { // > sum
             j--;
         }
     }
     System.out.println(result);
 } // out: [-1:10, 0:9, 2:7]

题2、给定一个int数组，一个数sum，求数组中和为sum的任意3个数的组合

 @Test
 public void test_find3() {
     int[] arr = { -1, 0, 2, 3, 4, 7, 8, 9, 10 };
     int sum = 9;
     Arrays.sort(arr);
     Set<ThreeTuple<Integer, Integer, Integer>> result = new LinkedHashSet<>();

     for (int i = 0; i < arr.length; i++) {
         int temp = arr[i];
         swap(arr, i, 0);
         int start = 1;
         int end = arr.length - 1;
         while (start < end) {
             if (arr[start] + arr[end] == sum - temp) {
                 int[] threeNum = {temp, arr[start], arr[end]};
                 Arrays.sort(threeNum);
                 result.add(new ThreeTuple<>(threeNum[0], threeNum[1], threeNum[2]));
                 start++;
                 end--;
             } else if (arr[start] + arr[end] > sum - temp) {
                 end--;
             } else {
                 start++;
             }
         }
         swap(arr, i, 0); // 还原
     }
     System.out.println(result);
 } // out: [-1:0:10, -1:2:8, -1:3:7, 0:2:7, 2:3:4]

 private void swap(int[] a, int i, int j) {
     int temp = a[i];
     a[i] = a[j];
     a[j] = temp;
 }

上面两题用到的元组类：

 class TwoTuple<A, B> {
     public final A first;
     public final B second;

     public TwoTuple(A a, B b) {
         this.first = a;
         this.second = b;
     }

     @Override public String toString() {
         return first + ":" + second;
     }

     @Override
     public int hashCode() {
         final int prime = 31;
         int result = 1;
         result = prime * result + ((first == null) ? 0 : first.hashCode());
         result = prime * result + ((second == null) ? 0 : second.hashCode());
         return result;
     }

     @Override
     public boolean equals(Object obj) {
         if (this == obj)
             return true;
         if (obj == null)
             return false;
         if (getClass() != obj.getClass())
             return false;
         TwoTuple<?, ?> other = (TwoTuple<?, ?>) obj;
         if (first == null) {
             if (other.first != null)
                 return false;
         } else if (!first.equals(other.first))
             return false;
         if (second == null) {
             if (other.second != null)
                 return false;
         } else if (!second.equals(other.second))
             return false;
         return true;
     }
 }

TwoTuple<A, B>

 class ThreeTuple<A, B, C> extends TwoTuple<A, B> {
     public final C third;

     public ThreeTuple(A a, B b, C c) {
         super(a, b);
         this.third = c;
     }

     @Override public String toString() {
         return super.toString() + ":" + third;
     }

     @Override
     public int hashCode() {
         final int prime = 31;
         int result = super.hashCode();
         result = prime * result + ((third == null) ? 0 : third.hashCode());
         return result;
     }

     @Override
     public boolean equals(Object obj) {
         if (this == obj)
             return true;
         if (!super.equals(obj))
             return false;
         if (getClass() != obj.getClass())
             return false;
         ThreeTuple<?, ?, ?> other = (ThreeTuple<?, ?, ?>) obj;
         if (third == null) {
             if (other.third != null)
                 return false;
         } else if (!third.equals(other.third))
             return false;
         return true;
     }
 }

ThreeTuple<A, B, C>

题3、给定一个int正整数数组，一个数sum，求数组中和为sum的k个数的组合有多少种（k任意）。

另开一博客讨论这个问题。

指路：【算法习题】正整数数组中和为sum的任意个数的组合数——待完成