feat: add go solution to lc problem: No.0315 (doocs#847)

No.0315.Count of Smaller Numbers After Self

Author: ifmagic

solution/0300-0399/0315.Count of Smaller Numbers After Self/README.md

@@ -73,6 +73,8 @@
 -   线段树的每个叶子节点代表一个长度为 1 的元区间 `[x, x]`；
 -   对于每个内部节点 `[l, r]`，它的左儿子是 `[l, mid]`，右儿子是 `[mid + 1, r]`, 其中 `mid = ⌊(l + r) / 2⌋` (即向下取整)。
 
+**方法三：归并排序**
+
 <!-- tabs:start -->
 
 ### **Python3**
@@ -523,6 +525,67 @@ func countSmaller(nums []int) []int {
 }
 ```
 
+归并排序：
+
+```go
+type Pair struct {
+	val   int
+	index int
+}
+
+var (
+	tmp   []Pair
+	count []int
+)
+
+func countSmaller(nums []int) []int {
+	tmp, count = make([]Pair, len(nums)), make([]int, len(nums))
+	array := make([]Pair, len(nums))
+	for i, v := range nums {
+		array[i] = Pair{val: v, index: i}
+	}
+	sorted(array, 0, len(array)-1)
+	return count
+}
+
+func sorted(arr []Pair, low, high int) {
+	if low >= high {
+		return
+	}
+	mid := low + (high-low)/2
+	sorted(arr, low, mid)
+	sorted(arr, mid+1, high)
+	merge(arr, low, mid, high)
+}
+
+func merge(arr []Pair, low, mid, high int) {
+	left, right := low, mid+1
+	idx := low
+	for left <= mid && right <= high {
+		if arr[left].val <= arr[right].val {
+			count[arr[left].index] += right - mid - 1
+			tmp[idx], left = arr[left], left+1
+		} else {
+			tmp[idx], right = arr[right], right+1
+		}
+		idx++
+	}
+	for left <= mid {
+		count[arr[left].index] += right - mid - 1
+		tmp[idx] = arr[left]
+		idx, left = idx+1, left+1
+	}
+	for right <= high {
+		tmp[idx] = arr[right]
+		idx, right = idx+1, right+1
+	}
+	// 排序
+	for i := low; i <= high; i++ {
+		arr[i] = tmp[i]
+	}
+}
+```
+
 ### **...**
 
 ```