week3 solution

Author: hoyeongkwak

combination-sum/hoyeongkwak.ts

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+function combinationSum(candidates: number[], target: number): number[][] {
+  const dp: number[][][] = Array(target + 1).fill(null).map(() => [])
+  dp[0] = [[]]
+
+  for (let i = 1; i <= target; i++){
+    for (const num of candidates) {
+        if (i - num >= 0 && dp[i - num].length > 0) {
+            for (const combo of dp[i - num]) {
+                if (combo.length === 0 || num >= combo[combo.length - 1]) {
+                    dp[i].push([...combo, num])
+                }
+            }
+        }
+    }
+  }
+  return dp[target]
+};

decode-ways/hoyeongkwak.ts

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+/* 
+    time complexity : O(n)
+    space complexity : O(n)
+*/
+function numDecodings(s: string): number {
+    const memo = new Map<number, number>()
+    const decode = (index: number): number => {
+        if (index === s.length) return 1
+        if (s[index] === '0') return 0
+        if (memo.has(index)) return memo.get(index)
+        
+        let ways = decode(index + 1)
+        if (index + 1 < s.length && (s[index] === '1' || (s[index] === '2' && parseInt(s[index + 1]) <= 6))) {
+            ways += decode(index + 2)
+        }
+        memo.set(index, ways)
+        return ways
+    }
+    return decode(0)
+}

maximum-subarray/hoyeongkwak.ts

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+/* 
+    time complexity : O(n)
+    space complexity : O(1)
+*/
+function maxSubArray(nums: number[]): number {
+    let maxSum = -Infinity
+    let currSum = 0
+
+    for (let i = 0; i < nums.length; i++) {
+        currSum += nums[i]
+
+        if (currSum > maxSum) {
+            maxSum = currSum
+        }
+
+        if (currSum < 0) {
+            currSum = 0
+        }
+    }
+    return maxSum
+};

number-of-1-bits/hoyeongkwak.ts

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+/* 
+    time complexity : O(log n)
+    space complexity : O(1)
+*/
+function hammingWeight(n: number): number {   
+    let count = 0
+    while ( n != 0) {
+        count += n & 1
+        n >>>= 1
+    }
+    return count
+
+    /* 
+    time complexity : O(log n)
+    space complexity : O(log n)
+    */
+    // const twoBits = n.toString(2)
+    // const bitCount = twoBits.split('').filter((s) => s === '1').length
+    // return bitCount
+};

valid-palindrome/hoyeongkwak.ts

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+/*
+time complexity : O(n)
+space complexity : O(n)
+*/
+function isPalindrome(s: string): boolean {
+    if (s.length === 1) return true
+    const splitS = s.replace(/[^a-zA-Z0-9]/g, '').toLowerCase()
+    const revS = splitS.split('').reverse().join('')
+    return splitS === revS
+};