Buggy python submission (verdict: Runtime Error):
def main():
    N, M = map(int, input().split())
    S = [i == "B" for i in input()]
    ab = [list(map(int, input().split())) for _ in [0]*M]
    graph = [set() for _ in [0]*N]
    [graph[a-1].add(b-1) for a, b in ab]
    [graph[b-1].add(a-1) for a, b in ab]

    status = [[0, 0] for _ in [0]*N]

    for i in range(N):
        for j in g[i]:
            status[i][S[j]] += 1

    que = []
    for i, (p, q) in enumerate(status):
        if (p == 0) ^ (q == 0):
            que.append(i)

    while que:
        i = que.pop()
        c = S[i]
        for j in graph[i]:
            if i != j:
                graph[j].remove(i)
            status[j][c] -= 1
            p, q = status[j]
            if (p == 0) ^ (q == 0):
                que.append(j)
        g[i] = set()

    for i in g:
        if i:
            print("Yes")
            return
    print("No")


main()


Fixed submission:
def main():
    from sys import stdin
    input = stdin.readline
    n, m = map(int, input().split())
    s = [i == "B" for i in input()]
    ab = [list(map(int, input().split())) for _ in [0]*m]
    g = [set() for _ in [0]*n]
    [g[a-1].add(b-1) for a, b in ab]
    [g[b-1].add(a-1) for a, b in ab]

    g2 = [[0, 0] for _ in [0]*n]

    for i in range(n):
        for j in g[i]:
            g2[i][s[j]] += 1

    ss = []
    for i, (j, k) in enumerate(g2):
        if (j == 0 and k > 0) or (j > 0 and k == 0):
            ss.append(i)

    while ss:
        i = ss.pop()
        c = s[i]
        for j in g[i]:
            if i != j:
                g[j].remove(i)
            g2[j][c] -= 1
            p, q = g2[j]
            if (p == 0 and q > 0) or (p > 0 and q == 0):
                ss.append(j)
        g[i] = set()

    for i in g:
        if len(i) > 0:
            print("Yes")
            return
    print("No")


main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    from sys import stdin
    input=stdin.readline
    from copy import deepcopy
    n, m = map(int, input().split())
    s = [i == "B" for i in input()]
    ab = [list(map(int, input().split())) for _ in [0]*m]
    g = [set() for _ in [0]*n]
    [g[a-1].add(b-1) for a, b in ab]
    [g[b-1].add(a-1) for a, b in ab]

    g2 = [[0, 0] for _ in [0]*n]

    for i in range(n):
        for j in g[i]:
            g2[i][s[j]] += 1

    ss = []
    for i, (j, k) in enumerate(g2):
        if (j == 0 and k > 0) or (j > 0 and k == 0):
            ss.append(i)

    while ss:
        i = ss.pop()
        c = s[i]
        for j in deepcopy(g[i]):
            g[j].remove(i)
            g2[j][c] -= 1
            p, q = g2[j]
            if (p == 0 and q > 0) or (p > 0 and q == 0):
                ss.append(j)
        g[i] = set()

    for i in g:
        if len(i) > 0:
            print("Yes")
            return
    print("No")


main()


Fixed submission:
def main():
    from sys import stdin
    input = stdin.readline
    n, m = map(int, input().split())
    s = [i == "B" for i in input()]
    ab = [list(map(int, input().split())) for _ in [0]*m]
    g = [set() for _ in [0]*n]
    [g[a-1].add(b-1) for a, b in ab]
    [g[b-1].add(a-1) for a, b in ab]

    g2 = [[0, 0] for _ in [0]*n]

    for i in range(n):
        for j in g[i]:
            g2[i][s[j]] += 1

    ss = []
    for i, (j, k) in enumerate(g2):
        if (j == 0 and k > 0) or (j > 0 and k == 0):
            ss.append(i)

    while ss:
        i = ss.pop()
        c = s[i]
        for j in g[i]:
            if i != j:
                g[j].remove(i)
            g2[j][c] -= 1
            p, q = g2[j]
            if (p == 0 and q > 0) or (p > 0 and q == 0):
                ss.append(j)
        g[i] = set()

    for i in g:
        if len(i) > 0:
            print("Yes")
            return
    print("No")


main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    from copy import deepcopy
    n, m = map(int, input().split())
    s = [i == "B" for i in input()]
    ab = [list(map(int, input().split())) for _ in [0]*m]
    g = [set() for _ in [0]*n]
    [g[a-1].add(b-1) for a, b in ab]
    [g[b-1].add(a-1) for a, b in ab]

    g2 = [[0, 0] for _ in [0]*n]

    for i in range(n):
        for j in g[i]:
            g2[i][s[j]] += 1

    ss = []
    for i, (j, k) in enumerate(g2):
        if (j == 0 and k > 0) or (j > 0 and k == 0):
            ss.append(i)

    while ss:
        i = ss.pop()
        c = s[i]
        for j in deepcopy(g[i]):
            g[j].remove(i)
            g2[j][c] -= 1
            p, q = g2[j]
            if (p == 0 and q > 0) or (p > 0 and q == 0):
                ss.append(j)
        g[i] = set()

    for i in g:
        if len(i) > 0:
            print("Yes")
            return
    print("No")


main()


Fixed submission:
def main():
    from sys import stdin
    input = stdin.readline
    n, m = map(int, input().split())
    s = [i == "B" for i in input()]
    ab = [list(map(int, input().split())) for _ in [0]*m]
    g = [set() for _ in [0]*n]
    [g[a-1].add(b-1) for a, b in ab]
    [g[b-1].add(a-1) for a, b in ab]

    g2 = [[0, 0] for _ in [0]*n]

    for i in range(n):
        for j in g[i]:
            g2[i][s[j]] += 1

    ss = []
    for i, (j, k) in enumerate(g2):
        if (j == 0 and k > 0) or (j > 0 and k == 0):
            ss.append(i)

    while ss:
        i = ss.pop()
        c = s[i]
        for j in g[i]:
            if i != j:
                g[j].remove(i)
            g2[j][c] -= 1
            p, q = g2[j]
            if (p == 0 and q > 0) or (p > 0 and q == 0):
                ss.append(j)
        g[i] = set()

    for i in g:
        if len(i) > 0:
            print("Yes")
            return
    print("No")


main()