Bellman-Ford算法能够找到图中的负环。负环是指一条的权值之和为负数的环路。如果在n次迭代后仍然存在松弛操作，那么说明图中存在负环。我们可以在遍历图的过程中记录每个节点到源节点的最短路径长度，如果在第n次松弛操作后仍然存在更新，那么说明图中存在负环。代码示例如下：

def bellman_ford(graph, start):
    dist = {node: float('inf') for node in graph}
    dist[start] = 0
    for _ in range(len(graph) - 1):
        for node in graph:
            for neighbor in graph[node]:
                distance = dist[node] + graph[node][neighbor]
                if distance < dist[neighbor]:
                    dist[neighbor] = distance
    for node in graph:
        for neighbor in graph[node]:
            if dist[node] + graph[node][neighbor] < dist[neighbor]:
                return True
    return False

当执行bellman_ford(graph, start)时，如果返回True，说明图中存在负环；如果返回False，则说明图中不存在负环。