Add problem 541

Author: avhn

algorithms/541/README.md

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+## 541. Reverse String II
+
+Given a string and an integer k, you need to reverse the first k characters for every 2k
+characters counting from the start of the string. If there are less than k characters
+left, reverse all of them. If there are less than 2k but greater than or equal to k
+characters, then reverse the first k characters and left the other as original.
+
+**Example:**
+<pre>
+<b>Input:</b> s = abcdefg, k = 2
+<b>Output:</b> bacdfeg
+</pre>
+
+**Restrictions:**
+
+1. The string consists of lower English letters only.
+2. Length of the given string and k will in the range [1, 10000]

algorithms/541/solution.go

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+package main
+
+import "strings"
+
+// Time and space complexity, O(n) where n is length of the string
+func reverseStr(s string, k int) string {
+	var sb strings.Builder
+	l := len(s)
+	for i := 0; i < l; {
+		if l < i+k {
+			k = l - i
+		}
+		for j := i + k - 1; i <= j; j-- {
+			sb.WriteByte(s[j])
+		}
+		e := i + 2*k
+		if l < e {
+			e = l
+		}
+		sb.WriteString(s[i+k : e])
+		i = e
+	}
+	return sb.String()
+}