The Phase-Shift Network

A signal travels through a network of N servers (0 to N-1). Some servers are 'Phase-Inverters'. When a signal enters a Phase-Inverter server, its polarity flips (Positive becomes Negative, Negative becomes Positive). Non-inverters keep the polarity the same. Given a start server, a target server, and the initial polarity (Positive), determine if there exists a path that reaches the target with a Negative polarity. You cannot visit the same server twice with the same polarity. Input Format: - Line 1: N, M, Start, Target. - Line 2: N integers (0 or 1). 1 means the server is a Phase-Inverter, 0 means it is not. - Next M lines: U V (a directed connection from U to V). Output Format: - 'YES' if target is reachable with Negative polarity, else 'NO'. Example: Input: 3 2 0 2 0 1 0 0 1 1 2 Output: YES (Explanation: Path 0 (Pos) -> 1 (flips to Neg) -> 2 (stays Neg).)