Problem p03682 (python)

Fixed submission:
'''
https://atcoder.jp/contests/abc065/tasks/arc076_b
最小全域木を考える際にある頂点について他の頂点への最小コストを
考えて繋げていくのは間違い(全域木になる保証がない)
考えたいのはあくまでも全辺のコスト

今回はxとyでそれぞれソートして隣り合う2点のコストのみを考えれば良い
'''
def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10000000)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate
    #from itertools import product
    from bisect import bisect_left,bisect_right
    import heapq
    from math import floor, ceil
    #from operator import itemgetter

    #inf = 10**17
    #mod = 1000000007

    N = int(input())
    xy = []
    for i in range(N):
        x,y = map(int, input().split())
        xy.append([x, y, i])
    x_sort = sorted(xy, key=lambda x: x[0])
    y_sort = sorted(xy, key=lambda x: x[1])

    #[cost, 頂点1, 頂点2]
    cost = []
    for i in range(N-1):
        cost.append([abs(x_sort[i][0]-x_sort[i+1][0]), x_sort[i][2], x_sort[i+1][2]])
        cost.append([abs(y_sort[i][1]-y_sort[i+1][1]), y_sort[i][2], y_sort[i+1][2]])
    cost = sorted(cost, key=lambda x: x[0])

    def find(x):
        if par[x] < 0:
            return x
        else:
            par[x] = find(par[x])
            return par[x]
    
    def unite(x,y):
        x = find(x)
        y = find(y)
        if x == y:
            return False
        else:
            if par[x] > par[y]:
                x,y = y,x
            par[x] += par[y]
            par[y] = x
            return True
    
    def same(x,y):
        return find(x) == find(y)

    par = [-1]*N

    res = 0
    for c, v1, v2 in cost:
        if not same(v1, v2):
            unite(v1, v2)
            res += c
    print(res)


if __name__ == '__main__':
    main()

Buggy submission (verdict: Runtime Error):
def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10000000)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate
    #from itertools import product
    from bisect import bisect_left,bisect_right
    import heapq
    from math import floor, ceil
    #from operator import itemgetter

    #inf = 10**17
    #mod = 1000000007

    N = int(input())
    xy = []
    for i in range(N):
        x,y = map(int, input().split())
        xy.append([x, y, i])
    x_sort.sort()
    #x_sort = sorted(xy, key=lambda x: x[0])
    y_sort = sorted(xy, key=lambda x: x[1])

    #[cost, 頂点1, 頂点2]
    cost = []
    for i in range(N-1):
        cost.append([x_sort[i+1][0]-x_sort[i][0], x_sort[i][2], x_sort[i+1][2]])
        cost.append([y_sort[i+1][1]-y_sort[i][1], y_sort[i][2], y_sort[i+1][2]])
    cost.sort()
    #cost = sorted(cost, key=lambda x: x[0])

    def find(x):
        if par[x] < 0:
            return x
        else:
            par[x] = find(par[x])
            return par[x]
    
    def unite(x,y):
        x = find(x)
        y = find(y)
        if x == y:
            return False
        else:
            if par[x] > par[y]:
                x,y = y,x
            par[x] += par[y]
            par[y] = x
            return True
    
    def same(x,y):
        return find(x) == find(y)

    par = [-1]*N

    res = 0
    for c, v1, v2 in cost:
        if not same(v1, v2):
            unite(v1, v2)
            res += c
    print(res)


if __name__ == '__main__':
    main()