go: add «2780. Minimum Index of a Valid Split»

URL:	https://leetcode.com/problems/minimum-index-of-a-valid-split/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/minimum-index-of-a-valid-split.go

@@ -0,0 +1,47 @@
+package main
+
+func minimumIndex(nums []int) int {
+	majorityElement := func() int {
+		dom, count := nums[0], 0
+
+		for _, x := range nums {
+			if x == dom {
+				count++
+			} else {
+				count--
+			}
+
+			if count == 0 {
+				dom = x
+				count = 1
+			}
+		}
+
+		return dom
+	}
+	dom := majorityElement()
+
+	// get frequency
+	domCount := 0
+	for _, x := range nums {
+		if x == dom {
+			domCount++
+		}
+	}
+
+	// Check for split
+	count := 0
+	for i, x := range nums {
+		if x == dom {
+			count++
+		}
+
+		remaining := domCount - count
+		if 2*count > i+1 && 2*remaining > len(nums)-i-1 {
+			return i
+		}
+	}
+
+	// couldn't find a valid split
+	return -1
+}