combine

combination/Combinations.java

@@ -0,0 +1,41 @@
+package Algorithms.combination;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Combinations {
+    public List<List<Integer>> combine(int n, int k) {
+        List<List<Integer>> ret = new ArrayList<List<Integer>>();
+
+        if (n == 0 || k == 0) {
+            return ret;
+        }
+
+        List<Integer> path = new ArrayList<Integer>();
+
+        combine(n, k, path, ret, 1);
+
+        return ret;
+    }
+
+    // index means the position which I can choose from.
+    // for example: when n = 4, 
+    // 1, 2, 3, 4, and index = 1, means now I can choose a number from 1 - 4
+    // 
+    public void combine(int n, int k, List<Integer> path, List<List<Integer>> ret, int index) {
+        if (0 == k) {
+            ret.add(new ArrayList<Integer>(path));
+            return;
+        }
+
+        // 注意这里的终止条件.
+        // For example: N = 4的时候，K = 2的时候,
+        // 这里还有2个可以取值，那么 1, 2, 3, 4中index最多可以从3取值，也就是4-2+1.
+        // 就是 n - k + 1
+        for (int i = index; i <= n - k + 1; i++) {
+            path.add(i);
+            combine(n, k - 1, path, ret, i + 1);
+            path.remove(path.size() - 1);
+        }
+    }
+}