go: add «670. Maximum Swap»

URL:	https://leetcode.com/problems/maximum-swap/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/maximum-swap.go

@@ -0,0 +1,46 @@
+package main
+
+func maximumSwap(num int) int {
+	fromInt := func() (int, []int) {
+		digits := make([]int, 9)
+
+		i := 0
+		for ; num > 0; i++ {
+			digits[i] = num % 10
+			num /= 10
+		}
+
+		return i, digits
+	}
+
+	toInt := func(digits []int) int {
+		result := 0
+
+		for i := len(digits) - 1; i >= 0; i-- {
+			result *= 10
+			result += digits[i]
+		}
+
+		return result
+	}
+
+	length, digits := fromInt()
+
+	for i := length - 1; i > 0; i-- {
+		max_index := i - 1
+		for j := i - 1; j >= 0; j-- {
+			if digits[j] > digits[i] && digits[j] >= digits[max_index] {
+				max_index = j
+			}
+		}
+
+		// found bigger digit that can be swapped
+		if digits[max_index] > digits[i] {
+			digits[max_index], digits[i] = digits[i], digits[max_index]
+			return toInt(digits)
+		}
+	}
+
+	// no swaps happened
+	return toInt(digits)
+}