feat(solutions): add combinations

Author: WindomZ

README.md

@@ -25,6 +25,7 @@
 
 | # | Problem | Solution | Difficulty | Single Repetition Duration | LeetCode Run Time |
 | ---: | :----- | :--------: | :----------: | ----------: | ----------: |
+|77|[Combinations][Solutions-77]|[WindomZ][Solutions-77-golang]|Medium|[336 ns/op/6 test cases][Solutions-77-Test]|246 ms|
 |75|[Sort Colors][Solutions-75]|[WindomZ][Solutions-75-golang]|Medium|[13.4 ns/op/6 test cases][Solutions-75-Test]|3 ms|
 |74|[Search a 2D Matrix][Solutions-74]|[WindomZ][Solutions-74-golang]|Medium|[32.5 ns/op/6 test cases][Solutions-74-Test]|16 ms|
 |73|[Set Matrix Zeroes][Solutions-73]|[WindomZ][Solutions-73-golang]|Medium|[22.0 ns/op/6 test cases][Solutions-73-Test]|42 ms|
@@ -128,6 +129,9 @@ Welcome to report bugs, suggest ideas and discuss on [issues page](https://githu
 ### Support
 If you like it then you can put a :star:Star on it.
 
+[Solutions-77]:https://leetcode.com/problems/combinations/
+[Solutions-77-golang]:solutions/combinations/combinations.go
+[Solutions-77-Test]:solutions/combinations/combinations_test.go#L32
 [Solutions-75]:https://leetcode.com/problems/sort-colors/
 [Solutions-75-golang]:solutions/sort_colors/sortcolors.go
 [Solutions-75-Test]:solutions/sort_colors/sortcolors_test.go#L35

solutions/combinations/README.md

@@ -0,0 +1,24 @@
+# 77. Combinations
+
+[Description](https://leetcode.com/problems/combinations/description/) | 
+[Discuss](https://leetcode.com/problems/combinations/discuss/) | 
+[Solution](https://leetcode.com/problems/combinations/solution/)
+
+## Description
+
+Given two integers _n_ and _k_, return all possible combinations of _k_ numbers out of 1 ... _n_.
+
+For example,  
+
+If _n_ = 4 and _k_ = 2, a solution is:
+
+```
+[
+  [2,4],
+  [3,4],
+  [2,3],
+  [1,2],
+  [1,3],
+  [1,4],
+]
+```

solutions/combinations/combinations.go

@@ -0,0 +1,26 @@
+package combinations
+
+func combine(n int, k int) [][]int {
+	if k <= 0 {
+		return [][]int{}
+	}
+	rs := make([][]int, 0, k)
+	combineNext(&rs, make([]int, 0, k), 1, n, k)
+	return rs
+}
+
+func combineNext(rs *[][]int, r []int, s, n, k int) {
+	if k == 0 {
+		t := make([]int, len(r))
+		for k, v := range r {
+			t[k] = v
+		}
+		*rs = append(*rs, t)
+		return
+	}
+	for i := s; i <= n; i++ {
+		r = append(r, i)
+		combineNext(rs, r, i+1, n, k-1)
+		r = r[:len(r)-1]
+	}
+}

solutions/combinations/combinations_test.go

@@ -0,0 +1,46 @@
+package combinations
+
+import (
+	"testing"
+
+	"github.com/WindomZ/testify/assert"
+)
+
+func Test_combine(t *testing.T) {
+	assert.Equal(t, [][]int{}, combine(1, 0))
+	assert.Equal(t, [][]int{
+		{1},
+	}, combine(1, 1))
+	assert.Equal(t, [][]int{
+		{1, 2},
+	}, combine(2, 2))
+	assert.Equal(t, [][]int{
+		{1, 2},
+		{1, 3},
+		{2, 3},
+	}, combine(3, 2))
+	assert.Equal(t, [][]int{
+		{1, 2},
+		{1, 3},
+		{1, 4},
+		{2, 3},
+		{2, 4},
+		{3, 4},
+	}, combine(4, 2))
+}
+
+func Benchmark_combine(b *testing.B) {
+	b.StopTimer()
+	b.ReportAllocs()
+	b.StartTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			combine(1, 0)
+			combine(1, 1)
+			combine(2, 2)
+			combine(3, 2)
+			combine(4, 2)
+			combine(4, 3)
+		}
+	})
+}