Lua单调栈：下一个更大元素的定位

-- 错误：逐个向后扫，最坏平方级
for i, v in ipairs(a) do
    for j = i + 1, #a do
        if a[j] > v then
            out[i] = j
            break
        end
    end
end

local function nextGreater(a)
    local out, stack = {}, {}
    for i, v in ipairs(a) do
        out[i] = 0
        while #stack > 0 and a[stack[#stack]] < v do
            out[table.remove(stack)] = i
        end
        stack[#stack + 1] = i
    end
    return out
end
local label = panel:getChildByName("ngText")
label:setString(table.concat(nextGreater({2, 7, 3, 9, 6}), ","))