Merge branch 'master' of https://github.com/cxjwin/go-algocasts

leetcode/leetcode493/reverse_pairs.go

@@ -0,0 +1,90 @@
+package leetcode493
+
+func reversePairsBruteForce(nums []int) int {
+	if nums == nil || len(nums) == 0 {
+		return 0
+	}
+
+	n := len(nums)
+	count := 0
+
+	for i := 0; i < n; i++ {
+		for j := i + 1; j < n; j++ {
+			if nums[i] > nums[j]*2 {
+				count++
+			}
+		}
+	}
+
+	return count
+}
+
+func merge(nums []int, low int, mid int, high int) {
+	m, n := mid-low+1, high-mid
+	a1 := make([]int, m)
+	for i := 0; i < m; i++ {
+		a1[i] = nums[low+i]
+	}
+	a2 := make([]int, n)
+	for i := 0; i < n; i++ {
+		a2[i] = nums[mid+1+i]
+	}
+
+	i, j, k := 0, 0, low
+	for i < m && j < n {
+		if a1[i] < a2[j] {
+			nums[k] = a1[i]
+			i++
+		} else {
+			nums[k] = a2[j]
+			j++
+		}
+		k++
+	}
+	for i < m {
+		nums[k] = a1[i]
+		i++
+		k++
+	}
+	for j < n {
+		nums[k] = a2[j]
+		j++
+		k++
+	}
+}
+
+func count(nums []int, low int, mid int, high int) int {
+	cnt := 0
+	i, j := low, mid+1
+	for i <= mid && j <= high {
+		if nums[i] <= nums[j]*2 {
+			i++
+		} else {
+			cnt += (mid - i + 1)
+			j++
+		}
+	}
+	return cnt
+}
+
+func sortAndCount(nums []int, low int, high int) int {
+	if low >= high {
+		return 0
+	}
+
+	cnt := 0
+	mid := low + (high-low)/2
+	cnt += sortAndCount(nums, low, mid)
+	cnt += sortAndCount(nums, mid+1, high)
+	cnt += count(nums, low, mid, high)
+	merge(nums, low, mid, high)
+
+	return cnt
+}
+
+func reversePairs(nums []int) int {
+	if nums == nil || len(nums) == 0 {
+		return 0
+	}
+	return sortAndCount(nums, 0, len(nums)-1)
+}

leetcode/leetcode493/reverse_pairs_test.go

@@ -0,0 +1,14 @@
+package leetcode493
+
+import "testing"
+
+func TestReversePairs(t *testing.T) {
+	nums := []int{1, 3, 2, 3, 1}
+	if reversePairsBruteForce(nums) != 2 {
+		t.Error("a : 1,3,2,3,1 -> 2")
+	}
+
+	if reversePairs(nums) != 2 {
+		t.Error("b : 1,3,2,3,1 -> 2")
+	}
+}