【高级技巧】中缀转后缀：表达式求值的栈实现封装

local PREC = { ["+"] = 1, ["-"] = 1, ["*"] = 2, ["/"] = 2 }
local function toPostfix(tokens)
    local out, ops = {}, {}
    for _, tk in ipairs(tokens) do
        if PREC[tk] then
            while #ops > 0 and PREC[ops[#ops]] >= PREC[tk] do
                out[#out + 1] = table.remove(ops)
            end
            ops[#ops + 1] = tk
        else
            out[#out + 1] = tk
        end
    end
    while #ops > 0 do
        out[#out + 1] = table.remove(ops)
    end
    return out
end
local function evalPostfix(post)
    local st = {}
    for _, tk in ipairs(post) do
        if tonumber(tk) then
            st[#st + 1] = tonumber(tk)
        else
            local b = table.remove(st)
            local a = table.remove(st)
            if tk == "+" then
                st[#st + 1] = a + b
            elseif tk == "*" then
                st[#st + 1] = a * b
            end
        end
    end
    return st[1]
end

local post = toPostfix({ "3", "+", "4", "*", "2" })
print(evalPostfix(post))