[moonjonghoo] Week 03 solutions

combination-sum/moonjonghoo.js

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+function combinationSum(candidates, target) {
+  const result = [];
+
+  // 1. 정렬: 가지치기를 위한 필수
+  candidates.sort((a, b) => a - b);
+
+  function backtrack(startIndex, path, remaining) {
+    if (remaining === 0) {
+      result.push([...path]);
+      return;
+    }
+
+    for (let i = startIndex; i < candidates.length; i++) {
+      const current = candidates[i];
+
+      // 2. 가지치기
+      if (current > remaining) break;
+
+      // 3. 현재 값 선택
+      path.push(current);
+      backtrack(i, path, remaining - current); // i로 재귀 호출: 같은 수 중복 사용 가능
+      path.pop(); // 4. 백트래킹
+    }
+  }
+
+  backtrack(0, [], target);
+  return result;
+}

decode-ways/moonjonghoo.js

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+function numDecodings(s) {
+  const n = s.length;
+
+  if (s[0] === "0") return 0;
+
+  const dp = new Array(n + 1).fill(0);
+  dp[0] = 1;
+  dp[1] = 1;
+
+  for (let i = 2; i <= n; i++) {
+    const oneDigit = Number(s.slice(i - 1, i));
+    const twoDigits = Number(s.slice(i - 2, i));
+
+    if (oneDigit >= 1 && oneDigit <= 9) {
+      dp[i] += dp[i - 1];
+    }
+
+    if (twoDigits >= 10 && twoDigits <= 26) {
+      dp[i] += dp[i - 2];
+    }
+  }
+
+  return dp[n];
+}

valid-palindrome/moonjonghoo.js

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+/**
+ * @param {string} s
+ * @return {boolean}
+ */
+
+var isPalindrome = function (s) {
+  // 1. 영숫자만 남기고, 소문자로 변환
+  let cleaned = s.toLowerCase().replace(/[^a-z0-9]/g, "");
+
+  // 2. 뒤집기
+  let reversed = cleaned.split("").reverse().join("");
+
+  // 3. 비교
+  return cleaned === reversed;
+};
+
+isPalindrome("A man, a plan, a canal: Panama");