add 784 script.

Author: everthis

784-letter-case-permutation.js

@@ -0,0 +1,27 @@
+/**
+ * @param {string} S
+ * @return {string[]}
+ */
+const letterCasePermutation = function(S) {
+    let res = []
+    backtrack(res, "", 0, S)
+    return res
+};
+
+const backtrack = (res, sol, depth, S) => {
+    if (depth == S.length) {
+        res.push(sol)
+        return
+    }
+    const c = S[depth]
+    if ("1234567890".indexOf(c) != - 1) {
+        backtrack(res, sol+c, depth + 1, S)
+    } else {
+        backtrack(res, sol+c.toLowerCase(), depth + 1, S)
+        backtrack(res, sol+c.toUpperCase(), depth + 1, S)
+    }
+}
+
+console.log(letterCasePermutation("a1b2")) 
+console.log(letterCasePermutation("3z4")) 
+console.log(letterCasePermutation("12345"))