def updateMatrix(matrix):
    matrix_length = len(matrix)
    if matrix_length < 1:
        return matrix
    row_length = len(matrix[0])
    queue = []
    visited = set()
    for i in range(matrix_length):
        for j in range(row_length):
            if matrix[i][j] == 0:
                queue.append((i, j))#找到所有的0结点
                visited.add((i, j))#将0节点看作已经被更新的节点visited
    while queue:
        i, j = queue.pop(0)
        for x, y in [(i + 1, j), (i - 1, j), (i, j + 1), (i, j - 1)]:  # 下上右左4个节点，0的上下左右到0的距离都为1
            if 0 <= x < matrix_length and 0 <= y < row_length and (x, y) not in visited:  # x,y范围区间在矩阵边界值以内,且未被更新
                matrix[x][y] = matrix[i][j] + 1
                visited.add((x, y))
                queue.append((x, y))
    return matrix