|
| 1 | +/** |
| 2 | + * @see {@link https://leetcode.com/problems/find-pivot-index/description/?envType=study-plan&id=level-1 724. Find Pivot Index} |
| 3 | + */ |
| 4 | + |
| 5 | +/** |
| 6 | + * EXPLAIN |
| 7 | + * Pivot index is EXCLUSIVE, not to be counted in comparative sums |
| 8 | + * [1,7,3,6,5,6] has pivot index 3 since 1+7+3 === 5+6 |
| 9 | + * The 6 at pivot index 3 is NOT counted |
| 10 | + * |
| 11 | + * First index and last index sums are always 0! |
| 12 | + * [2,1,-1] has PI 0; 2@0 not counted, so it's 1 + -1 |
| 13 | + * [1,-1,2] also has PI 0, but on right edge, same reason |
| 14 | + * Return -1 if no pivot index found |
| 15 | + * |
| 16 | + * Flow |
| 17 | + * [1,3,7,2,2] |
| 18 | + * l = 0 r = 15 - arr[0] = 14 |
| 19 | + * l = 0 + arr[0] = 1 r = 14 - arr[1] = 11 |
| 20 | + * l = 1 + arr[1] = 4 r = 11 - arr[2] = 4 |
| 21 | + */ |
| 22 | + |
| 23 | +/** |
| 24 | + * APPROACH |
| 25 | + * Create leftSum, rightSum vals; default left to 0 |
| 26 | + * |
| 27 | + * START LOOP at second val |
| 28 | + * running sum to get first right sum |
| 29 | + * END LOOP |
| 30 | + * |
| 31 | + * if l === r, return 0 (left edge) |
| 32 | + * |
| 33 | + * START 2ND LOOP at second val |
| 34 | + * i - 1 added to leftVal |
| 35 | + * subtract curr val from right sum |
| 36 | + * check left===right; if so, return i |
| 37 | + * END 2ND LOOP |
| 38 | + * |
| 39 | + * no PI found, return -1 |
| 40 | + */ |
| 41 | + |
| 42 | +/** |
| 43 | + * @param {number[]} nums |
| 44 | + * @return {number} |
| 45 | + */ |
| 46 | +var pivotIndex = function (nums) { |
| 47 | + let left = 0; |
| 48 | + let right = 0; |
| 49 | + |
| 50 | + // 1st Loop to establish right sum |
| 51 | + for (let i = 1; i < nums.length; i++) { |
| 52 | + const currVal = nums[i]; |
| 53 | + right += currVal; |
| 54 | + } |
| 55 | + |
| 56 | + // Left-edge check |
| 57 | + if (left === right) { |
| 58 | + return 0; |
| 59 | + } |
| 60 | + |
| 61 | + // Begin looping through array to add/subtract from left/right totals |
| 62 | + for (let j = 1; j < nums.length; j++) { |
| 63 | + // Add prevVal to left |
| 64 | + left += nums[j - 1]; |
| 65 | + |
| 66 | + // Subtract currVal from right |
| 67 | + right -= nums[j]; |
| 68 | + |
| 69 | + // Equality check |
| 70 | + if (left === right) { |
| 71 | + return j; |
| 72 | + } |
| 73 | + } |
| 74 | + |
| 75 | + // Loop ended, no PI found |
| 76 | + return -1; |
| 77 | +}; |
| 78 | + |
| 79 | +/** |
| 80 | + * COMPLEXITY |
| 81 | + * TIME O(N) - b/c of one loop |
| 82 | + * SPACE O(1) - left and right vars are constant integers |
| 83 | + */ |
| 84 | + |
| 85 | +/** |
| 86 | + * OPTIMIZE |
| 87 | + * |
| 88 | + * calc sum of right first |
| 89 | + * LOOP |
| 90 | + * grab curr num |
| 91 | + * subtract curr from right |
| 92 | + * test (left === right) ? return index |
| 93 | + * add curr to left |
| 94 | + * END LOOP |
| 95 | + * |
| 96 | + * no match found; return -1 |
| 97 | + */ |
| 98 | + |
| 99 | +/** |
| 100 | + * READ 01:25 |
| 101 | + * EXPLAIN 02:00 03:25 |
| 102 | + * APPROACH 22:35 26:01 |
| 103 | + * CODE 11:36 37:38 |
| 104 | + * TEST 1st Attempt |
| 105 | + * OPTIMIZE |
| 106 | + */ |
0 commit comments