Minimum Height Difference Between Distant Peaks

Select two sufficiently distant peaks with minimum height difference, using sorting or indexed scanning to satisfy the distance constraint. Examples Example 1: Input: heights = [1,5,3,9,7], gap = 2 Output: 2 Explanation: Valid pairs (|i - j| >= 2): (0,2)|1-3|=2, (0,3)|1-9|=8, (0,4)|1-7|=6, (1,3)|5-9|=4, (1,4)|5-7|=2, (2,4)|3-7|=4. Minimum is 2. Example 2: Input: heights = [10,20,30,40], gap = 3 Output: 30 Explanation: Only (0,3) satisfies |i - j| >= 3, giving |10 - 40| = 30. Example 3: Input: heights = [1,2,3], gap = 5 Output: -1 Explanation: No index pair is 5 apart in a length-3 array, so return -1. Constraints 1 <= heights.length <= 10^4 0 <= heights[i] <= 10^9 1 <= gap <= 10^4