[kimyoung] WEEK 04 Solutions

longest-consecutive-sequence/kimyoung.js

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+var longestConsecutive = function (nums) { // sorting approach
+    if (!nums.length) return 0;
+    let set = new Set(nums);
+    let sorted = [...set].sort((a, b) => a - b);
+    let longestSeq = 0;
+    let currSeq = 1;
+
+    for (let i = 1; i < sorted.length; i++) { // loop through sorted list to find sequence
+        if (sorted[i - 1] + 1 === sorted[i]) {
+            currSeq++;
+        } else {
+            longestSeq = Math.max(longestSeq, currSeq); // compare sequence to figure out the longest
+            currSeq = 1;
+        }
+    }
+
+    return Math.max(longestSeq, currSeq);
+};
+
+// time - O(nlong) using sort
+// space - O(n) store nums in set
+
+
+// TODO - try O(n) TC approach

missing-number/kimyoung.js

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+var missingNumber = function (nums) { // brute force approach
+    let missingNumber;
+    for (let i = 0; i < nums.length; i++) {
+        if (nums.indexOf(i) === -1) missingNumber = i;
+    }
+    return missingNumber === undefined ? nums.length : missingNumber;
+};
+
+// time - O(n^2) finding the index of i while iterating through nums
+// splace - O(1) no extra space needed other than missingNumber which just stores the result
+
+var missingNumber = function (nums) { // set approach 
+    let set = new Set(nums);
+    for (let i = 0; i < nums.length + 1; i++) {
+        if (!set.has(i)) return i;
+    }
+};
+
+// time - O(n) looping through nums to find the missing number
+// splace - O(n) creating a set of nums
+
+var missingNumber = function (nums) { // mathematical approach
+    const len = nums.length
+    const expectedSum = len * (len + 1) / 2;
+    const actualSum = nums.reduce((acc, el) => acc += el, 0);
+    return expectedSum - actualSum;
+};
+
+// time - O(n) reduce method on actualSum
+// space - O(1) extra space irrelevant to input

valid-palindrome/kimyoung.js

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+var isPalindrome = function (s) {
+    let left = 0, right = s.length - 1;
+    while (left < right) {
+        while (left < right && !isAlphaNumeric(s[left])) { // increment the left index until it's an alphanumeric character
+            left++;
+        }
+        while (left < right && !isAlphaNumeric(s[right])) { // decrement the right index until it's an alphanumeric character
+            right--;
+        }
+        if (s[left++].toLowerCase() !== s[right--].toLowerCase()) return false; // compare the two string values, if different return false;
+    }
+    return true;
+};
+
+function isAlphaNumeric(char) { // use ASCII code to findout if char is alphanumeric or not
+    const asciiCode = char.charCodeAt(0);
+    return (asciiCode >= 65 && asciiCode <= 90) ||
+        (asciiCode >= 97 && asciiCode <= 122) ||
+        (asciiCode >= 48 && asciiCode <= 57)
+}
+
+// time - O(n) iterate through the string once
+// space - O(1) no extra space created