If and have different depths, bring the deeper one up to the depth of the shallower one. Compute depth[v] for all nodes (via BFS or DFS).
If depth[a] > depth[b], jump up by depth[a] - depth[b] steps. Now and are at the same depth. Use binary lifting to perform the jump in time. After this step, and are at the same level and you can begin searching for their meeting point.