decode-ways solution

Author: togo26

decode-ways/togo26.js

@@ -0,0 +1,76 @@
+/**
+ * @param {string} s
+ * @return {number}
+ */
+
+// TLE
+// var numDecodings = function(s) {
+//     const codeMap = {};
+//     for (let i = 1; i <= 26; i++) {
+//         codeMap[i] = String.fromCharCode(64 + i);
+//     }
+
+//     const memo = {};
+//     let count = 0;
+//     function go(string, start) {
+//         if (start >= string.length) {
+//             count++;
+//             return;
+//         }
+
+//         const firstDigit = string[start];
+//         const secondDigit = string[start + 1];
+//         if (firstDigit === "0") return;
+
+//         if (codeMap[firstDigit]) {
+//             go(string, start + 1);
+//         }
+
+//         const twoDigits = firstDigit + secondDigit;
+//         if (Number(twoDigits) <= 26 && codeMap[twoDigits]) {
+//             go(string, start + 2);
+//         }
+//     }
+
+//     go(s, 0, "");
+
+//     return count;
+// };
+
+// TC: O(n) -> string 길이만큼
+// SC: O(n) -> string 길이만큼 스택 생성
+var numDecodings = function (s) {
+  const codeMap = {};
+  for (let i = 1; i <= 26; i++) {
+    codeMap[i] = String.fromCharCode(64 + i);
+  }
+
+  const memo = {};
+
+  function go(start) {
+    if (start >= s.length) return 1;
+    if (memo[start] !== undefined) return memo[start];
+
+    const firstDigit = s[start];
+    const secondDigit = s[start + 1];
+
+    if (firstDigit === '0') return 0;
+
+    let ways = 0;
+
+    if (codeMap[firstDigit]) {
+      ways += go(start + 1);
+    }
+
+    const twoDigits = firstDigit + secondDigit;
+    if (Number(twoDigits) <= 26 && codeMap[twoDigits]) {
+      ways += go(start + 2);
+    }
+
+    memo[start] = ways;
+
+    return ways;
+  }
+
+  return go(0);
+};