Create 2083-substrings-that-begin-and-end-with-the-same-letter.js

Author: everthis

2083-substrings-that-begin-and-end-with-the-same-letter.js

@@ -0,0 +1,28 @@
+/**
+ * @param {string} s
+ * @return {number}
+ */
+const numberOfSubstrings = function(s) {
+  const hash = {}
+  const n = s.length
+  for(let i = 0; i < n; i++) {
+    const ch = s[i]
+    if(hash[ch] == null) hash[ch] = []
+    hash[ch].push(i)
+  }
+  
+  let res = 0
+  const keys = Object.keys(hash)
+  keys.forEach(k => {
+    res += helper(k)
+  })
+  
+  return res
+  
+  
+  function helper(k) {
+    const arr = hash[k]
+    const len = arr.length
+    return len * (len + 1) / 2
+  }
+};