feat: more exercises from chapter 7

Author: damianobacci

chapter-7/ex3.ts

@@ -0,0 +1,18 @@
+function findMissingLetter(str: string): string {
+    let hashMap = new Map<string, boolean>()
+
+    for (const char of str) {
+        if (hashMap.has(char)) {
+            continue
+        }
+        hashMap.set(char, true)
+    }
+
+    const alphabet: string = 'abcdefghijklmnopqrstuvwxyz'
+    for (const letter of alphabet) {
+        if (!hashMap.has(letter)) {
+            return letter
+        }
+    }
+    return 'No missing letter found'
+}

chapter-7/ex4.ts

@@ -0,0 +1,18 @@
+function returnFirstNonDuplicated(str: string): string {
+    const hashMap = new Map<string, boolean>()
+
+    for (const char of str) {
+        if (hashMap.has(char)) {
+            hashMap.set(char, false)
+        } else {
+            hashMap.set(char, true)
+        }
+    }
+
+    for (const [char, isUnique] of hashMap) {
+        if (isUnique) {
+            return char;
+        }
+    }
+    return 'No non-duplicated character found'
+}