Search for a Range

Given an array of integersnumssorted in ascending order, find the starting and ending position of a giventargetvalue.

Your algorithm's runtime complexity must be in the order of O(log n).

If the target is not found in the array, return[-1, -1].

Example 1:

Input: nums = [5,7,7,8,8,10], target = 8
Output: [3,4]

Example 2:

Input: nums = [5,7,7,8,8,10], target = 6
Output: [-1,-1]

Solution

Linear scan.

class Solution {
    public int[] searchRange(int[] nums, int target) {
        int[] targetRange = {-1, -1};

        // find the index of the leftmost appearance of `target`.
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target) {
                targetRange[0] = i;
                break;
            }
        }

        // if the last loop did not find any index, then there is no valid range
        // and we return [-1, -1].
        if (targetRange[0] == -1) {
            return targetRange;
        }

        // find the index of the rightmost appearance of `target` (by reverse
        // iteration). it is guaranteed to appear.
        for (int j = nums.length-1; j >= 0; j--) {
            if (nums[j] == target) {
                targetRange[1] = j;
                break;
            }
        }

        return targetRange;
    }
}

Using binary search.

class Solution {
    // returns leftmost (or rightmost) index at which `target` should be
    // inserted in sorted array `nums` via binary search.
    private int extremeInsertionIndex(int[] nums, int target, boolean left) {
        int lo = 0;
        int hi = nums.length;

        while (lo < hi) {
            int mid = (lo+hi)/2;
            if (nums[mid] > target || (left && target == nums[mid])) {
                hi = mid;
            }
            else {
                lo = mid+1;
            }
        }

        return lo;
    }

    public int[] searchRange(int[] nums, int target) {
        int[] targetRange = {-1, -1};

        int leftIdx = extremeInsertionIndex(nums, target, true);

        // assert that `leftIdx` is within the array bounds and that `target`
        // is actually in `nums`.
        if (leftIdx == nums.length || nums[leftIdx] != target) {
            return targetRange;
        }

        targetRange[0] = leftIdx;
        targetRange[1] = extremeInsertionIndex(nums, target, false) - 1;

        return targetRange;
    }
}