Wireless Frequency Deadzone

Given a BST of occupied radio frequencies and a target frequency F, find the 'deadzone width'. The deadzone is the gap between the smallest frequency in the tree strictly greater than F and the largest frequency in the tree strictly less than F. If a boundary doesn't exist, use F as that boundary value. Output the absolute difference. Example: Input: 100 50 150 25 75 60 Output: 25 Explanation: Predecessor of 60 is 50. Successor of 60 is 75. Gap: 75 - 50 = 25.