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String/859.md

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+## Buddy Strings
+
+#### Description
+
+[link](https://leetcode.com/problems/buddy-strings/)
+
+---
+
+#### Solution
+
+- See Code
+
+---
+
+#### Code
+
+O(n)
+
+```python
+class Solution:
+    def buddyStrings(self, A: str, B: str) -> bool:
+        if len(A) <= 1 or len(B) <= 1 or len(A) != len(B):
+            return False
+        if A == B:
+            return len(set(A)) < len(A)
+        i = 0
+        while A[i] == B[i]: i += 1
+        for j in range(i + 1, len(A)):
+            if A[j] == B[i] and A[i] == B[j]:
+                A = A[:i] + A[j] + A[i+1:j] + A[i] + A[j+1:]
+        return A == B
+```