# [C (gcc)], 62 bytes

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 o(char*s){return!s[1]|!s[2]||(*s-s[1])*(s[1]-s[2])>=0&o(s+1);}

Assumes signed `char`.

[Try it online!]

[C (gcc)]: https://gcc.gnu.org/
[Try it online!]: https://tio.run/nexus/c-gcc#VYzBDoIwEETvfMVag2kRjHgU5UeUQ20LNNFiuuUEfDu2qDHuYSc7O/PmjoqW2wTZYJXrrVnhJa9Gvw/VONIEs3CzhAbJgsvK837TUdzmrJjmtTbi3ksFJ3RSd7u2jCJtHDiFDhY0eDYMEfh5Wv@qKYnxCLG8GpICpuBZjBXR9C4@uDb0WwgUSvhNSFU3xId@ZlMrKW783/wkvZCFOL8A "C (gcc) – TIO Nexus"