import numpy as np
from collections import deque
 
def bfs_finder(d, start):
    sorter = np.argsort(d[:, 0])
    done = set()
    todo = deque([start])
    output = np.empty_like(d)
    pos = 0
    while todo:
        key = todo.popleft()
        if key in done:
            continue
        done.add(key)
        left = np.searchsorted(d[:, 0], key, 'left', sorter)
        if left >= d.shape[0] or d[sorter[left], 0] != key:
            continue
        right = np.searchsorted(d[:, 0], key, 'right', sorter)
        next = pos + right - left
        output[pos:next, :] = d[sorter[left:right], :]
        todo.extend(output[pos:next, 1])
        pos = next
    return output
 
arr = np.array([[1, 7], [2, 0], [2, 1], [2, 3], [3, 4], [3, 5], [5, 6]])
print(bfs_finder(arr, 2))

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