  We present a unified viewpoint of proximal point method (PPM), primal-dual
hybrid gradient (PDHG) and alternating direction method of multipliers (ADMM)
for solving convex-concave primal-dual problems. This viewpoint shows the
equivalence of these three algorithms upto a norm change, and it leads to a
four-line simple proof of their $\mathcal O(1/k)$ ergodic rates. The simple
proof technique is not limited to these three algorithms, but can also be
utilized to analyze related algorithms, such as gradient descent, linearized
PDHG, inexact algorithms, just to name a few.
