34. Find First and Last Position of Element in Sorted Array

solutions/findFirstAndLastPositionOfElementInSortedArray.test.ts

@@ -0,0 +1,15 @@
+import searchRange from "./findFirstAndLastPositionOfElementInSortedArray";
+
+const TEST_CASES = new Map<[number[], number], [number, number]>([
+  [[[5, 7, 7, 8, 8, 10], 8], [3, 4]],
+  [[[5, 7, 7, 8, 8, 10], 6], [-1, -1]],
+  [[[2, 2], 2], [0, 1]]
+]);
+
+describe("34. Find First and Last Position of Element in Sorted Array", () => {
+  for (const [args, expected] of TEST_CASES) {
+    test(`when [${args[0].join(", ")}], ${args[1]}`, () => {
+      expect(searchRange(args[0], args[1])).toEqual(expected);
+    });
+  }
+});

solutions/findFirstAndLastPositionOfElementInSortedArray.ts

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+// 34. Find First and Last Position of Element in Sorted Array
+// https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/
+export default function searchRange(
+  nums: number[],
+  target: number
+): [number, number] {
+  // binary search to find the first index
+  let from = 0;
+  let to = nums.length;
+
+  // loop until the search range shrinks to be from + 2 === to
+  while (to - from >= 3) {
+    const middle = Math.floor((from + to) / 2);
+
+    if (nums[middle] >= target) {
+      to = middle + 1;
+    } else {
+      from = middle;
+    }
+  }
+
+  // nums[from] or nums[to - 1] may point the target value
+  // otherwise, there's no target value
+  let first: number;
+
+  if (nums[from] === target) {
+    first = from;
+  } else if (nums[to - 1] === target) {
+    first = to - 1;
+  } else {
+    return [-1, -1];
+  }
+
+  // binary search to find the last index
+  from = first >= 0 ? first : 0;
+  to = nums.length;
+
+  // do the same binary search for the last index
+  while (to - from >= 3) {
+    const middle = Math.floor((from + to) / 2);
+
+    if (nums[middle] <= target) {
+      from = middle;
+    } else {
+      to = middle + 1;
+    }
+  }
+
+  let last: number;
+
+  // pick to-value first because the values both from and to point could be target
+  if (nums[to - 1] === target) {
+    last = to - 1;
+  } else if (nums[from] === target) {
+    last = from;
+  }
+
+  return [first, last!];
+}