303. Range Sum Query - Immutable（python+cpp）

题目：

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.
Example:

Given nums = [-2, 0, 3, -5, 2, -1]
sumRange(0, 2) -> 1 
sumRange(2, 5) -> -1 
sumRange(0, 5) -> -3

Note:
You may assume that the array does not change.
There are many calls to sumRange function.

解释：
当然可以直接用求和函数，但是特别慢。其实这种题目一般需要用一个数组储存前n项和，然后在求rangeSum的时候直接相减即可，前n项和是一个很经典的方法，如sums[i]保存包括nums[i]的前i+1项和。
直接用求和函数，python代码：

class NumArray:

    def __init__(self, nums):
        """
        :type nums: List[int]
        """
        self.nums=nums
    def sumRange(self, i, j):
        """
        :type i: int
        :type j: int
        :rtype: int
        """
        return sum(self.nums[i:j+1])
# Your NumArray object will be instantiated and called as such:
# obj = NumArray(nums)
# param_1 = obj.sumRange(i,j)

前n项和解法，python代码：

class NumArray(object):

    def __init__(self, nums):
        """
        :type nums: List[int]
        """
        n=len(nums)
        self.stepsum=[0]*n
        if n>0:
            self.stepsum[0]=nums[0]
        for i in xrange(1,n):
            self.stepsum[i]=self.stepsum[i-1]+nums[i]
        
    def sumRange(self, i, j):
        """
        :type i: int
        :type j: int
        :rtype: int
        """
        return self.stepsum[j]-(self.stepsum[i-1] if i >0 else 0)
       
# Your NumArray object will be instantiated and called as such:
# obj = NumArray(nums)
# param_1 = obj.sumRange(i,j)

c++代码：

class NumArray {
public:
    vector<int>sums;
    NumArray(vector<int> nums) {
        int n=nums.size();
        sums.assign(n,-1);
        if (n>0)
            sums[0]=nums[0];
        for (int i=1;i<nums.size();i++)
        {
            sums[i]=sums[i-1]+nums[i];
        }
    }
    
    int sumRange(int i, int j) {
        return sums[j]- (i>0?sums[i-1]:0);
    }
};

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(i,j);
 */

总结：
c++中给一个已经声明好的vector 赋值：
vector<int> sums;
sums.assign(nums.begin(),nums.end()); （这个比下面那句速度更快）
sums=vector<int>(nums.begin(),nums.end());