add: solutions for 3 problems

Author: HerrineKim

climbing-stairs/HerrineKim.js

@@ -0,0 +1,17 @@
+// 시간복잡도: O(n)
+// 공간복잡도: O(1)
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var climbStairs = function (n) {
+  if (n <= 2) return n;
+  let prev1 = 1, prev2 = 2;
+  for (let i = 3; i <= n; i++) {
+    let curr = prev1 + prev2;
+    prev1 = prev2;
+    prev2 = curr;
+  }
+  return prev2;
+};

decode-ways/HerrineKim.js

@@ -0,0 +1,26 @@
+// 시간복잡도: O(n)
+// 공간복잡도: O(n)
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var numDecodings = function (s) {
+  const memo = {};
+
+  const helper = (index) => {
+    if (index === s.length) return 1;
+    if (s[index] === '0') return 0;
+    if (memo[index] !== undefined) return memo[index];
+
+    let ways = helper(index + 1);
+    if (index < s.length - 1 && parseInt(s.slice(index, index + 2)) <= 26) {
+      ways += helper(index + 2);
+    }
+
+    memo[index] = ways;
+    return ways;
+  };
+
+  return helper(0);
+};

valid-anagram/HerrineKim.js

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+// 시간복잡도: O(n)
+// 공간복잡도: O(1)
+
+/**
+ * @param {string} s
+ * @param {string} t
+ * @return {boolean}
+ */
+var isAnagram = function (s, t) {
+  if (s.length !== t.length) return false;
+
+  const charCount = {};
+
+  for (const char of s) {
+    charCount[char] = (charCount[char] || 0) + 1;
+  }
+
+  for (const char of t) {
+    if (!charCount[char]) {
+      return false;
+    }
+    charCount[char]--;
+  }
+
+  return true;
+};
+