A swing from left to right is only half a complete cycle ... the complete cycle is a swing from left to right, and back left again. This means the pendulum has a period of 2 seconds, not 1. That's the trick.
The period and the length of a pendulum are related by the equation
T = 2π√(L/g) and squaring both sides yields
T² = 2²π²L/g
T² = 4π²L/g
gT² = 4π²L
L = gT²/4π²
L = 9.8*2²/4π² = 9.8/π² = 0.99 m which is close to 1 m.