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Though it won't work for deadbolts, it will work for spring locks (the kind that's slanted and a part of a handle). Grab one that's cheap and you don't care about -- if it breaks, no harm, no foul. A bendy, laminated card works best. Force it between the lock and the frame, flush against the frame. Bend it back, forcing the lock back into the door, opening it. If there's no room between the lock and the frame, you can force it between the door and the frame above the lock, then swipe quickly downward while angling the card toward the frame. This is where a thicker, sturdier card may come in handy.
Use a credit card on a spring lock.