added problem 859

Author: SHA65536

problems/problem0859/buddy_strings.go

@@ -0,0 +1,38 @@
+package problem0859
+
+/*
+Given two strings s and goal, return true if you can swap two letters in s so the result is equal to goal, otherwise, return false.
+Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at s[i] and s[j].
+    For example, swapping at indices 0 and 2 in "abcd" results in "cbad".
+*/
+
+func buddyStrings(s string, goal string) bool {
+	// Different length - can't swap to fix
+	if len(s) != len(goal) {
+		return false
+	}
+	if s == goal {
+		// If they're both the same, we can only make them equal
+		// by swapping duplicate letters twice
+		var set uint32 // Save existence of letters as bits
+		for i := range s {
+			if set&(1<<(s[i]-'a')) > 0 {
+				// If we've already seen this letter, there are duplicates
+				return true
+			}
+			// Mark letter as seen
+			set |= (1 << (s[i] - 'a'))
+		}
+		return false
+	}
+	// Dif is an array of indexes where the strings differ
+	var dif = make([]int, 0, len(s))
+	for i := 0; i < len(s) && len(dif) <= 2; i++ {
+		if s[i] != goal[i] {
+			dif = append(dif, i)
+		}
+	}
+	return len(dif) == 2 && // We can only fix two differences
+		s[dif[0]] == goal[dif[1]] && // the differences should be the same letter
+		s[dif[1]] == goal[dif[0]] // and should be symmetrical
+}

problems/problem0859/buddy_strings_test.go

@@ -0,0 +1,37 @@
+package problem0859
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type TestCase struct {
+	S, Goal  string
+	Expected bool
+}
+
+var TestCases = []TestCase{
+	{"ab", "ba", true},
+	{"ab", "ab", false},
+	{"aa", "aa", true},
+}
+
+func TestBuddyStrings(t *testing.T) {
+	assert := assert.New(t)
+
+	for _, tc := range TestCases {
+		want := tc.Expected
+		got := buddyStrings(tc.S, tc.Goal)
+		assert.Equal(want, got, fmt.Sprintf("%+v", tc))
+	}
+}
+
+func BenchmarkBuddyStrings(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range TestCases {
+			buddyStrings(tc.S, tc.Goal)
+		}
+	}
+}

readme.md

@@ -295,6 +295,7 @@ Each problem is in it's own directory, with test files. There are helper package
 | 0844 | [Backspace String Compare](https://leetcode.com/problems/backspace-string-compare/) | [My Solution](./problems/problem0844) | ✅ |
 | 0848 | [Shifting Letters](https://leetcode.com/problems/shifting-letters) | [My Solution](./problems/problem0848) | ✅ |
 | 0858 | [Mirror Reflection](https://leetcode.com/problems/mirror-reflection) | [My Solution](./problems/problem0858) | ✅ |
+| 0859 | [Buddy Strings](https://leetcode.com/problems/buddy-strings) | [My Solution](./problems/problem0859) | ✅ |
 | 0864 | [Shortest Path to Get All Keys](https://leetcode.com/problems/shortest-path-to-get-all-keys) | [My Solution](./problems/problem0864) | ✅ |
 | 0867 | [Transpose Matrix](https://leetcode.com/problems/transpose-matrix) | [My Solution](./problems/problem0867) | ✅ |
 | 0869 | [Reordered Power of 2](https://leetcode.com/problems/reordered-power-of-2) | [My Solution](./problems/problem0869) | ✅ |