[hyer0705] WEEK 03 solutions

combination-sum/hyer0705.ts

@@ -0,0 +1,21 @@
+function combinationSum(candidates: number[], target: number): number[][] {
+  const results: number[][] = [];
+
+  function backtrack(currentIndex: number, sum: number, selected: number[]) {
+    if (sum > target) return;
+    if (sum === target) {
+      results.push([...selected]);
+      return;
+    }
+
+    for (let i = currentIndex; i < candidates.length; i++) {
+      selected.push(candidates[i]);
+      backtrack(i, sum + candidates[i], selected);
+      selected.pop();
+    }
+  }
+
+  backtrack(0, 0, []);
+
+  return results;
+}

decode-ways/hyer0705.ts

@@ -0,0 +1,25 @@
+function numDecodings(s: string): number {
+  const sLen = s.length;
+  const isValid = (s: string): boolean => {
+    if (s[0] === "0") return false;
+
+    return Number(s) > 0 && Number(s) <= 26;
+  };
+
+  if (sLen === 0) return 0;
+  if (s.length === 1) return isValid(s[0]) ? 1 : 0;
+
+  // dp[i]: i번째 위치까지 디코딩할 수 있는 방법의 수
+  const dp: number[] = Array(sLen).fill(0);
+  dp[0] = isValid(s[0]) ? 1 : 0;
+  dp[1] = (isValid(s[1]) ? dp[0] : 0) + (isValid(s.substring(0, 2)) ? 1 : 0);
+
+  for (let i = 2; i < sLen; i++) {
+    const singleDigitWays = isValid(s[i]) ? dp[i - 1] : 0;
+    const doubleDigitWays = isValid(s[i - 1] + s[i]) ? dp[i - 2] : 0;
+
+    dp[i] = singleDigitWays + doubleDigitWays;
+  }
+
+  return dp[sLen - 1];
+}

maximum-subarray/hyer0705.ts

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+function maxSubArray(nums: number[]): number {
+  const numsLen = nums.length;
+  let currentSum = nums[0];
+  let maxSum = nums[0];
+
+  for (let i = 1; i < numsLen; i++) {
+    currentSum = Math.max(currentSum + nums[i], nums[i]);
+    maxSum = Math.max(maxSum, currentSum);
+  }
+
+  return maxSum;
+}

number-of-1-bits/hyer0705.ts

@@ -0,0 +1,5 @@
+function hammingWeight(n: number): number {
+  const RADIX = 2;
+
+  return n.toString(RADIX).match(/1/g)?.length || 0;
+}

valid-palindrome/hyer0705.ts

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+function isPalindrome(s: string): boolean {
+  const converted = s.toLowerCase().replace(/[^a-z\d]/g, "");
+
+  let l = 0;
+  let r = converted.length - 1;
+
+  while (l < r) {
+    if (converted[l] !== converted[r]) return false;
+    l++;
+    r--;
+  }
+
+  return true;
+}