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Add problem 541

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	`1`	`+## 541. Reverse String II`
	`2`	`+`
	`3`	`+Given a string and an integer k, you need to reverse the first k characters for every 2k`
	`4`	`+characters counting from the start of the string. If there are less than k characters`
	`5`	`+left, reverse all of them. If there are less than 2k but greater than or equal to k`
	`6`	`+characters, then reverse the first k characters and left the other as original.`
	`7`	`+`
	`8`	`+Example:`
	`9`	`+<pre>`
	`10`	`+<b>Input:</b> s = abcdefg, k = 2`
	`11`	`+<b>Output:</b> bacdfeg`
	`12`	`+</pre>`
	`13`	`+`
	`14`	`+Restrictions:`
	`15`	`+`
	`16`	`+1. The string consists of lower English letters only.`
	`17`	`+2. Length of the given string and k will in the range [1, 10000]`

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	`1`	`+package main`
	`2`	`+`
	`3`	`+import "strings"`
	`4`	`+`
	`5`	`+// Time and space complexity, O(n) where n is length of the string`
	`6`	`+func reverseStr(s string, k int) string {`
	`7`	`+ var sb strings.Builder`
	`8`	`+ l := len(s)`
	`9`	`+ for i := 0; i < l; {`
	`10`	`+ if l < i+k {`
	`11`	`+ k = l - i`
	`12`	`+ }`
	`13`	`+ for j := i + k - 1; i <= j; j-- {`
	`14`	`+ sb.WriteByte(s[j])`
	`15`	`+ }`
	`16`	`+ e := i + 2*k`
	`17`	`+ if l < e {`
	`18`	`+ e = l`
	`19`	`+ }`
	`20`	`+ sb.WriteString(s[i+k : e])`
	`21`	`+ i = e`
	`22`	`+ }`
	`23`	`+ return sb.String()`
	`24`	`+}`

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