[togo26] WEEK 03 Solutions (#2723)

togo26 · parkhojeong · dahyeong-yun · web-flow · commit 80e2e1733138 · 2026-07-12T01:40:04.000+09:00
* contains-duplicate solution * two-sum solution * top-k-frequent-elements solution * longest-consecutive-sequence solution * house-robber solution * valid-anagram solution * climbing-stairs solution * product-of-array-except-self solution * 3sum solution * validate-binary-search-tree solution * Apply suggestion from @parkhojeong Co-authored-by: Hojeong Park <parkhj062@gmail.com> * valid-palindrome solution * number-of-1-bits solution * combination-sum solution * decode-ways solution * maximum-subarray solution * Update valid-palindrome/togo26.js Remove unnecessary regex construction Co-authored-by: polymorph <33543196+dahyeong-yun@users.noreply.github.com> * Update decode-ways/togo26.js Remove redundant code range validation Co-authored-by: polymorph <33543196+dahyeong-yun@users.noreply.github.com> --------- Co-authored-by: Hojeong Park <parkhj062@gmail.com> Co-authored-by: polymorph <33543196+dahyeong-yun@users.noreply.github.com>
diff --git a/combination-sum/togo26.js b/combination-sum/togo26.js
@@ -0,0 +1,35 @@
+/**
+ * @param {number[]} candidates
+ * @param {number} target
+ * @return {number[][]}
+ */
+
+// C = candidates, T = target, m = smallest candidate value
+// TC: O(C^(T/m)) -> 후보와 타겟 깊이 간 지수적 증가
+// SC: O(T/m) -> 재귀 스택 깊이 (*출력 결과는 복잡도 계산 제외)
+var combinationSum = function (candidates, target) {
+  const result = [];
+
+  function go(candi, combi, acc, start) {
+    if (acc === target) {
+      result.push(combi);
+      return;
+    }
+
+    for (let i = start; i < candi.length; i++) {
+      const newAcc = acc + candi[i];
+      const newCombi = [...combi, candi[i]];
+      if (newAcc > target) break;
+      go(candi, newCombi, newAcc, i);
+    }
+  }
+
+  go(
+    [...candidates].sort((a, b) => a - b), // 값 순서 보장으로 새로운 누적 값에서 조기 종료 가능. 오름차순에 따라 newAcc가 target을 넘어가면 이후의 후보들은 계산할 필요 없음.
+    [],
+    0,
+    0,
+  );
+
+  return result;
+};
diff --git a/decode-ways/togo26.js b/decode-ways/togo26.js
@@ -0,0 +1,73 @@
+/**
+ * @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;
+
+    ways += go(start + 1);
+    const twoDigits = firstDigit + secondDigit;
+    if (Number(twoDigits) <= 26) {
+      ways += go(start + 2);
+    }
+
+    memo[start] = ways;
+
+    return ways;
+  }
+
+  return go(0);
+};
diff --git a/maximum-subarray/togo26.js b/maximum-subarray/togo26.js
@@ -0,0 +1,53 @@
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+
+// TLE
+// var maxSubArray = function(nums) {
+//     if (nums.length <= 1) return nums[0];
+
+//     const sum = (arr) => arr.reduce((acc, cur) => {
+//         acc += cur;
+//         return acc;
+//     }, 0);
+
+//     let total = -Infinity;
+//     for (let i = 0; i < nums.length; i++) {
+//         for (let j = i; j < nums.length; j++) {
+//             const result = sum(nums.slice(i, j + 1));
+//             total = Math.max(total, result);
+//         }
+//     }
+
+//     return total;
+// };
+
+// TLE
+// var maxSubArray = function(nums) {
+//     if (nums.length <= 1) return nums[0];
+
+//     let total = -Infinity;
+//     for (let i = 0; i < nums.length; i++) {
+//         let acc = 0;
+//         for (let j = i; j < nums.length; j++) {
+//             acc += nums[j];
+//             total = Math.max(total, acc);
+//         }
+//     }
+
+//     return total;
+// };
+
+// TC: O(n) / SC: O(1)
+var maxSubArray = function (nums) {
+  let total = nums[0];
+  let acc = nums[0];
+
+  for (let i = 1; i < nums.length; i++) {
+    acc = Math.max(nums[i], acc + nums[i]); // 이전 누적값 선택 vs 현재값 선택 (카데인 알고리즘)
+    total = Math.max(total, acc); // 최대 합산 값 갱신
+  }
+
+  return total;
+};
diff --git a/number-of-1-bits/togo26.js b/number-of-1-bits/togo26.js
@@ -0,0 +1,64 @@
+/**
+ * @param {number} n
+ * @return {number}
+ */
+
+// 31 비트 범위는 상수로 취급
+
+// Log-based calculation
+// TC: O(1)  / SC: O(1)
+// 비트 제약이 없을 경우 -> TC: O(logn) / SC: O(logn)
+var hammingWeight = function (n) {
+  const BIT_RANGE = 31;
+  let bits = Array.from({ length: BIT_RANGE }).fill(0);
+
+  let remainder = n;
+  while (remainder > 0) {
+    const exp = Math.min(Math.floor(Math.log2(remainder)), BIT_RANGE - 1);
+    bits[exp] = true;
+    const value = remainder <= 1 ? 1 : Math.pow(2, exp);
+    remainder -= value;
+  }
+
+  return bits.filter(value => value).length;
+};
+
+// Log-based calculation without an array
+// TC: O(1) / SC: O(1)
+// 비트 제약이 없을 경우 -> TC: O(logn) / SC: O(1)
+var hammingWeight = function (n) {
+  const BIT_RANGE = 31;
+  let bitCount = 0;
+
+  let remainder = n;
+  while (remainder > 0) {
+    const exp = Math.min(Math.floor(Math.log2(remainder)), BIT_RANGE - 1);
+    const value = remainder <= 1 ? 1 : Math.pow(2, exp);
+    remainder -= value;
+    bitCount++;
+  }
+
+  return bitCount;
+};
+
+// Converting binary string with the JS feature
+// TC: O(1) / SC: O(1)
+// 비트 제약이 없을 경우 -> TC: O(logn) / SC: O(logn)
+var hammingWeight = function (n) {
+  const binaryString = n.toString(2);
+  return [...binaryString].filter(v => v === '1').length;
+};
+
+// Bit manipulation
+// TC: O(1) / SC: O(1)
+// 비트 제약이 없을 경우 -> TC: O(logn) / SC: O(logn)
+var hammingWeight = function (n) {
+  let count = 0;
+
+  while (n > 0) {
+    n = n & (n - 1);
+    count++;
+  }
+
+  return count;
+};
diff --git a/valid-palindrome/togo26.js b/valid-palindrome/togo26.js
@@ -0,0 +1,28 @@
+/**
+ * @param {string} s
+ * @return {boolean}
+ */
+// TC: O(n) / SC: O(n)
+var isPalindrome = function (s) {
+  const processed = s.toLowerCase().replaceAll(/[^a-z0-9]+/g, '');
+  const reversed = [...processed].reverse().join('');
+  return processed === reversed;
+};
+
+// TC: O(n) / SC: O(1)
+// With two pointers
+var isPalindrome = function (s) {
+  const nonAlphanumeric = /[^a-zA-Z0-9]+/;
+  let left = 0;
+  let right = s.length - 1;
+
+  while (left < right) {
+    while (left < right && nonAlphanumeric.test(s[left])) left++; // 단일 문자 테스트 O(1) 상수 취급
+    while (left < right && nonAlphanumeric.test(s[right])) right--;
+    if (s[left].toLowerCase() !== s[right].toLowerCase()) return false;
+    left++;
+    right--;
+  }
+
+  return true;
+};
