Accumulate求和算法:accumulate对要累加的元素类型一无所知，这个事实有两层含义。首先，调用该函数时必需传递一个初始值，否则，accumulate将不知道使用什么初始值。其次，容器内的元素类型必须与第三个实参的类型匹配，或者可转换为第三个实参的类型。在accumulate内部，第三个实参用作累加的起点；容器内的元素按顺序连续累加到综合之中。因此，必须能够将元素类型加到总和类型上。-The accumulate ignorant of to accumulate the element type, the fact that there are two meanings.First, call the function must pass an initial value, otherwise, the accumulate will don t know what to use the initial value.Second, the element type of the container must be compatible with the type of the third argument, or can be converted into the third argument types.Within the accumulate, the third argument used as the starting point of accumulation Continuous accumulation elements within a container in order to comprehensively.Therefore, must be able to add the element types to the combined type.