day 113: reverse sentence in place

day113/problem.go

@@ -1,6 +1,8 @@
 package day113
 
-import "strings"
+import (
+	"strings"
+)
 
 // ReverseWords reverses the word position in the string.
 // Runs in O(N) time and requires O(N) additional space.
@@ -15,5 +17,29 @@ func ReverseWords(str string) string {
 // ReverseWordsInPlace reverses the word position in the string.
 // Runs in O(N) time and requires O(1) additional space.
 func ReverseWordsInPlace(str string) string {
-	return "here world hello"
+	r := []rune(str)
+	for i := 0; i < len(r)/2; i++ {
+		r[i], r[len(r)-1-i] = r[len(r)-1-i], r[i]
+	}
+	var start, end int
+	for end <= len(r) {
+		if end == len(r) {
+			reverseInPlace(r, start, end)
+		} else if r[end] == ' ' {
+			reverseInPlace(r, start, end)
+			start = end
+			for r[start] == ' ' {
+				start++
+			}
+			end = start - 1
+		}
+		end++
+	}
+	return string(r)
+}
+
+func reverseInPlace(r []rune, start, end int) {
+	for i := start; i < (start+end)/2; i++ {
+		r[i], r[end-1-(i-start)] = r[end-1-(i-start)], r[i]
+	}
 }

day113/problem_test.go

@@ -6,6 +6,7 @@ var testcases = []struct {
 	input, expected string
 }{
 	{"hello world here", "here world hello"},
+	{"greetings from colorado", "colorado from greetings"},
 }
 
 func TestReverseWords(t *testing.T) {