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Add solution for Substrings That Begin and End With the Same Letter

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`@@ -100,6 +100,7 @@ Algorithm exercises from LeetCode implemented in Java v11.`
`100`	`100`
`101`	`101`	`### Prefix Sum`
`102`	`102`	`- Product of the Last K Numbers \| [Problem](https://leetcode.com/problems/product-of-the-last-k-numbers) \| [Solution](src/solutions/ProductOfNumbers.java)`
	`103`	`+- Substrings That Begin and End With the Same Letter \| [Problem](https://leetcode.com/problems/substrings-that-begin-and-end-with-the-same-letter) \| [Solution](src/solutions/SubstringsWithSameLetter.java)`
`103`	`104`
`104`	`105`	`### Recursion`
`105`	`106`	`- Closest Binary Search Tree Value \| [Problem](https://leetcode.com/problems/closest-binary-search-tree-value)`

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`@@ -0,0 +1,32 @@`
	`1`	`+package solutions;`
	`2`	`+`
	`3`	`+// [Problem] https://leetcode.com/problems/substrings-that-begin-and-end-with-the-same-letter`
	`4`	`+class SubstringsWithSameLetter {`
	`5`	`+ // Prefix sum`
	`6`	`+ // O(n) time, O(1) space`
	`7`	`+ public long numberOfSubstrings(String s) {`
	`8`	`+ int[] letterCounts = new int[26];`
	`9`	`+ long substringCount = 0;`
	`10`	`+ for (int i = 0; i < s.length(); i++) {`
	`11`	`+ char c = s.charAt(i);`
	`12`	`+ int letterCount = ++letterCounts[c - 'a'];`
	`13`	`+ substringCount += letterCount;`
	`14`	`+ }`
	`15`	`+ return substringCount;`
	`16`	`+ }`
	`17`	`+`
	`18`	`+ // Test`
	`19`	`+ public static void main(String[] args) {`
	`20`	`+ SubstringsWithSameLetter solution = new SubstringsWithSameLetter();`
	`21`	`+`
	`22`	`+ String input1 = "abcba";`
	`23`	`+ long expectedOutput1 = 7;`
	`24`	`+ long actualOutput1 = solution.numberOfSubstrings(input1);`
	`25`	`+ System.out.println("Test 1 passed? " + (expectedOutput1 == actualOutput1));`
	`26`	`+`
	`27`	`+ String input2 = "abacad";`
	`28`	`+ long expectedOutput2 = 9;`
	`29`	`+ long actualOutput2 = solution.numberOfSubstrings(input2);`
	`30`	`+ System.out.println("Test 2 passed? " + (expectedOutput2 == actualOutput2));`
	`31`	`+ }`
	`32`	`+}`

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