[reach0908] WEEK 03 solutions

combination-sum/reach0908.js

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+/**
+ * 시간복잡도 O(n^(T/m))
+ * 풀이방법: DFS 백트래킹
+ * 결과: 2ms
+ * @param {number[]} candidates
+ * @param {number} target
+ * @return {number[][]}
+ */
+const combinationSum = function (candidates, target) {
+  const result = [];
+  const nums = [];
+
+  const sortedCandidates = candidates.sort((a, b) => a - b);
+
+  function dfs(start, sum) {
+    if (sum === target) {
+      result.push([...nums]);
+    }
+
+    for (let i = start; i < sortedCandidates.length; i += 1) {
+      const num = sortedCandidates[i];
+
+      if (sum + num > target) break;
+
+      nums.push(num);
+      dfs(i, sum + num);
+      nums.pop();
+    }
+  }
+
+  dfs(0, 0);
+
+  return result;
+};

decode-ways/reach0908.js

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+/**
+ * 시간복잡도: O(n)
+ * 실행속도: 65ms
+ * @param {string} s
+ * @return {number}
+ */
+const numDecodings = function (s) {
+  const n = s.length;
+
+  const memo = new Array(n + 1).fill(0);
+
+  function dfs(i) {
+    if (i === n) {
+      return 1;
+    }
+
+    if (s[i] === "0") {
+      return 0;
+    }
+
+    if (memo[i]) {
+      return memo[i];
+    }
+
+    let res = dfs(i + 1);
+
+    if (i + 1 < n && Number(s.slice(i, i + 2) <= 26)) {
+      res += dfs(i + 2);
+    }
+
+    memo[i] = res;
+
+    return res;
+  }
+
+  return dfs[0];
+};
+
+/**
+ * 시간복잡도: O(n)
+ * 실행속도: 0~1ms
+ * @param {string} s
+ * @return {number}
+ */
+/**
+ * 시간 복잡도: O(n)
+ * 공간 복잡도: O(n)
+ */
+var numDecodings2 = function (s) {
+  const memo = new Map();
+  memo.set(s.length, 1);
+
+  function dfs(start) {
+    if (memo.has(start)) {
+      return memo.get(start);
+    }
+
+    if (s[start] === "0") {
+      memo.set(start, 0);
+    } else if (
+      start + 1 < s.length &&
+      parseInt(s.slice(start, start + 2)) < 27
+    ) {
+      memo.set(start, dfs(start + 1) + dfs(start + 2));
+    } else {
+      memo.set(start, dfs(start + 1));
+    }
+
+    return memo.get(start);
+  }
+
+  return dfs(0);
+};

maximum-subarray/reach0908.js

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+/**
+ * 시간복잡도: O(n)
+ * 시간: 4ms
+ * @param {number[]} nums
+ * @return {number}
+ */
+var maxSubArray = function (nums) {
+  let maxResult = nums[0];
+  let curResult = nums[0];
+
+  for (let i = 1; i < nums.length; i += 1) {
+    const num = nums[i];
+    // 지금까지 누적합 vs 현재 날짜
+    result = Math.max(curResult + num, num);
+    maxResult = Math.max(maxResult, curResult);
+  }
+
+  return maxResult;
+};

number-of-1-bits/reach0908.js

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+/**
+ * 시간복잡도: O(n)
+ * 공간복잡도: O(1)
+ * @param {number} n
+ * @return {number}
+ */
+var hammingWeight = function (n) {
+  let num = n;
+
+  let count = 0;
+
+  while (num > 0) {
+    const left = num % 2;
+
+    if (left === 1) {
+      count += 1;
+    }
+
+    num = Math.floor(num / 2);
+  }
+
+  return count;
+};

valid-palindrome/reach0908.js

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+/**
+ * 시간복잡도: O(N)
+ * 공간복잡도: O(1)
+ * @param {string} s
+ * @return {boolean}
+ */
+const isPalindrome = function (s) {
+  const parsedString = s
+    .trim()
+    .replace(" ", "")
+    .replace(/[^a-zA-Z0-9]/g, "")
+    .toLowerCase();
+
+  let left = 0;
+  let right = parsedString.length - 1;
+
+  while (left < right) {
+    if (parsedString[left] !== parsedString[right]) {
+      return false;
+    }
+
+    left += 1;
+    right -= 1;
+  }
+
+  return true;
+};