codeforces: B. Maximum Xor Secondary【Monotone Stack】

Analysis

When we fix a next largest element (every one is possible)
The largest element is either on the left or on the right
It is the first one on the left or right that can be found using a monotonic stackA number larger than the current next-largest value
Then record 2n times such a maximum-sub-largest combination

ac code

import sysinput = sys.stdin.readlinen = int(input())s = list(map(int, input().split()))ans = 0st = []# every v can be the second max, max on left or rightfor v in s:# decreasing stack# st[-1] second max, v max(max on right)while st and st[-1] <v:ans = max(st.pop() ^ v, ans)# v second max, st[-1] max(max on left)if st:ans = max(st[-1]^ v, ans)st.append(v)print(ans)

Summary

Monotone stack messing