Solve LeetCode 75 prefix sum problems

Author: THammami01

LeetCode-75/.vscode/launch.json

@@ -11,7 +11,7 @@
     //   "skipFiles": [
     //     "<node_internals>/**"
     //   ],
-    //   "program": "${workspaceFolder}\\LeetCode-75\\17-1493.js"
+    //   "program": "${workspaceFolder}\\LeetCode-75\\19-724.js"
     // }
   ]
 }

LeetCode-75/18-1732.ts

@@ -0,0 +1,14 @@
+// https://leetcode.com/problems/find-the-highest-altitude/description/?envType=study-plan-v2&envId=leetcode-75
+
+const largestAltitude = (gain: number[]): number => {
+  let highest = 0,
+    sum = 0;
+
+  // O(N) SOLUTION
+  for (let val of gain) {
+    sum += val;
+    if (sum > highest) highest = sum;
+  }
+
+  return highest;
+};

LeetCode-75/19-724.ts

@@ -0,0 +1,38 @@
+// https://leetcode.com/problems/find-pivot-index/description/?envType=study-plan-v2&envId=leetcode-75
+
+const pivotIndex = (nums: number[]): number => {
+  // O(N) SOLUTION, BUT UNACCEPTED; IT DOESN'T WORK PROPERLY WITH POSITIVE/NEGATIVE VALUES
+  // let i = 0,
+  //   j = nums.length - 1,
+  //   leftSum = 0,
+  //   rightSum = 0;
+
+  // while (i < j) {
+  //   const nextLeftSum = leftSum + nums[i];
+  //   const nextRightSum = rightSum + nums[j];
+
+  //   if (nextLeftSum > nextRightSum) {
+  //     // if (Math.abs(nextLeftSum) > Math.abs(nextRightSum)) {
+  //     rightSum = nextRightSum;
+  //     j--;
+  //   } else {
+  //     leftSum = nextLeftSum;
+  //     i++;
+  //   }
+  // }
+
+  // return rightSum === leftSum ? i : -1;
+
+  // O(N) SOLUTION
+  let i = 1,
+    leftSum = 0,
+    rightSum = nums.reduce((acc, val) => acc + val, 0) - nums[0];
+
+  while (i < nums.length && leftSum !== rightSum) {
+    leftSum += nums[i - 1];
+    rightSum -= nums[i];
+    i++;
+  }
+
+  return leftSum === rightSum ? i - 1 : -1;
+};

README.md

@@ -30,8 +30,8 @@
 
 ### Prefix Sum
 
-- [ ] 18. Find the Highest Altitude
-- [ ] 19. Find Pivot Index
+- [X] 18. Find the Highest Altitude
+- [X] 19. Find Pivot Index
 
 ### Hash Map / Set