Leetcode 1377. Frog Position After T Seconds

On a tree rooted at 1, compute the probability that a frog performing a non‑backtracking random walk—each second uniformly jumping to an unvisited neighbor or staying forever if none—will be at vertex target after t seconds. The core challenge is accounting for branching factors and the time constraint along the unique path from the root to the target (including the case the frog gets stuck at the target).