[hoyeongkwak] WEEK 02 Solutions

3sum/hoyeongkwak.ts

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+/*
+time complexity : O(n^2)
+space complexity : O(1)
+*/
+function threeSum(nums: number[]): number[][] {
+    const result: number[][] = []
+    const sortedNums = nums.sort((a, b) => a - b)
+
+    for(let i = 0; i < sortedNums.length - 2; i++) {
+        if (i > 0 && sortedNums[i] === sortedNums[i - 1]) continue
+        let low = i + 1
+        let high = sortedNums.length - 1
+        while (low < high) {
+            const threeSum = sortedNums[i] + sortedNums[low] + sortedNums[high]
+            if (threeSum < 0) {
+                low += 1
+            } else if (threeSum > 0) {
+                high -= 1
+            } else {
+                result.push([sortedNums[i], sortedNums[low], sortedNums[high]])
+                while (low < high && sortedNums[low] === sortedNums[low + 1]) low++
+                while (low < high && sortedNums[high] === sortedNums[high - 1]) high--
+
+                low += 1
+                high -= 1
+            }
+        }
+    }
+    return result
+}

climbing-stairs/hoyeongkwak.ts

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+/*
+시간복잡도 : O(n)
+공간복잡도 : O(1)
+*/
+function climbStairs(n: number): number {
+  if (n < 3) return n
+  let prev = 1
+  let curr = 2
+  for (let i = 0; i < n - 2; i++) {
+    const tempPrev = prev
+    prev = curr
+    curr = tempPrev + curr
+  }
+  return curr
+}

product-of-array-except-self/hoyeongkwak.ts

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+/*
+time complexity : O(n)
+space complexity : O(1)
+*/
+function productExceptSelf(nums: number[]): number[] {
+    const results = new Array(nums.length).fill(1) 
+    let before = 1
+    let after = 1
+    for (let i = 0; i < nums.length - 1; i++) {
+        before *= nums[i]
+        results[i + 1] *= before
+    }
+    for (let i = nums.length - 1; i > 0; i--) {
+        after *= nums[i]
+        results[i - 1] *= after
+    }
+
+    return results
+}

valid-anagram/hoyeongkwak.ts

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+function isAnagram(s: string, t: string): boolean {
+    // 시간복잡도 O(nlogn), 공간복잡도 O(n)
+    // const sSorted = s.split('').sort().join(',')
+    // const tSorted = t.split('').sort().join(',')
+    // return sSorted === tSorted
+
+    /*
+    시간복잡도 O(n), 공간복잡도 O(1)
+    */
+    if (s.length != t.length) return false
+    const count = new Array(26).fill(0)
+    for (let i = 0; i < s.length; i++) {
+        count[s.charCodeAt(i) - 97]++
+        count[t.charCodeAt(i) - 97]--
+    }
+    return count.every(c => c === 0)
+}

validate-binary-search-tree/hoyeongkwak.ts

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+/**
+ * Definition for a binary tree node.
+ * class TreeNode {
+ *     val: number
+ *     left: TreeNode | null
+ *     right: TreeNode | null
+ *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+ *         this.val = (val===undefined ? 0 : val)
+ *         this.left = (left===undefined ? null : left)
+ *         this.right = (right===undefined ? null : right)
+ *     }
+ * }
+ */
+/*
+time complexity : O(n)
+space complexity : O(n)
+*/
+function isValidBST(root: TreeNode | null): boolean {
+    let prev = -Infinity
+    let isValid = true
+    const inOrder = (node: TreeNode | null): void => {
+        if (!isValid || !node) return
+        inOrder(node.left)
+
+        if (prev >= node.val) {
+            isValid = false
+            return
+        }
+        prev = node.val
+        inOrder(node.right)
+    }
+    inOrder(root)
+    return isValid
+}