取下界的二分查找

-- 取下界的二分查找，未找到目标值，则取比目标值小的最近的数
-- 时间复杂度 log2n
 

function LC(a, obj)

    local max, start, end1 = -1, 1, #a
    while start <= end1 do
        local mid = math.modf(start+(end1-start)/2)
        if a[mid]>=obj then
            end1 = mid - 1
        else
            max = mid
            start = mid+1
        end
    end
    return a[max]
end

local a = {8,5,6,9,2,10,5,2,4,6}
print(LC(a, 4))