SRM 551

p250.
You know the optimal solution has to fix one pos unchanged. So try to fix each of the 0 to n-1 pos. For a fixed pos, search left and right to get the nearest same color letter until run out of maxSwap.
It took me a while to figure it out and my coding is rather slow.

p500.
A relatively easy 500 problem, observe that many people got 400+ points. However I could not find an algorithm during contest. This solution is due to liympanda.

We are to find how many 'Y' we need to remove to force a path from 0 to n-1. Generalize a bit, we need to find ans[0] to ans[n-1] where ans[i] is the min op to get from 0 to i. This leads to a DP + BFS solution.

First ans[0] = 0
Then we need to repeat at most n times.
In each iteration, we find min ans[i] such that i has not been used before, that is use[i] = 0
Then we use i to update all next such that color[i][next] is 'Y' and ans[next] can be improved. Notice that we also need to add an extra = k-1 if next is the k-th neighbor of i.

This is a bit like Dijkstra, each time we pull out the best node with min dist from src, then we use that to update dist of all other nodes. In the end we have dist of all nodes.