leetcode：Range Sum Query

看到leetcode这个网站，这个网站上的题都是一些经典的公司用来面试应聘者的面试题，感觉不错，就挑个最简单的题目来做：Range Sum Query

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.

 

题目的意思就是求数组中指定区间的和，并且求和函数会调用很多次。

直接Python写：

class NumArray(object):
    def __init__(self, nums):
        """
        initialize your data structure here.
        :type nums: List[int]
        """
        self.num = nums
        

    def sumRange(self, i, j):
        """
        sum of elements nums[i..j], inclusive.
        :type i: int
        :type j: int
        :rtype: int
        """
        reduce(lambda x,y:x+y,self.num[i:j+1])
        

 
if __name__ == '__main__':
    nums = [-2, 0, 3, -5, 2, -1]
    print sum(nums)
    numArray = NumArray(nums)
    numArray.sumRange(0, 1)
    numArray.sumRange(1, 2)

嗯，个人感觉挺简洁，运行后也没错误，复制代码提交，结果提示超时了~~~，再改代码

class NumArray(object):
    def __init__(self, nums):
        """
        initialize your data structure here.
        :type nums: List[int]
        """
        self.num = nums
        

    def sumRange(self, i, j):
        """
        sum of elements nums[i..j], inclusive.
        :type i: int
        :type j: int
        :rtype: int
        """
        sum(self.num[i:j+1])

这回应该没问题了吧，提交后还是提示超时，崩溃！上网查，网上都有答案了，因为sumRange函数要调用多次，所以不能每次调用都要计算，应该在初始化时计算好。。。唉，怎么没想到呢，改代码

class NumArray(object):
    def __init__(self, nums):
        """
        initialize your data structure here.
        :type nums: List[int]
        """
        self.sum = [0]
        for item in nums:
            self.sum.append(self.sum[-1]+item)
        

    def sumRange(self, i, j):
        """
        sum of elements nums[i..j], inclusive.
        :type i: int
        :type j: int
        :rtype: int
        """
        return self.sum[j + 1] - self.sum[i]

OK，成功！！！

附C++代码：

#include <stdio.h>
#include <vector>
using namespace std;

class NumArray {
public:
    vector <int> Mynum;
    NumArray(vector<int> &nums) {
        Mynum.push_back(0);
        vector<int>::iterator it;
        for(it=nums.begin();it!=nums.end();it++)
        {
            Mynum.push_back(Mynum.back()+ *it);
        }            
    }

    int sumRange(int i, int j) {
        return Mynum[j+1]-Mynum[i];    
    }
};

int main()
{
    vector <int> nums;
    nums.push_back(-2);
    nums.push_back(0);
    nums.push_back(3);
    nums.push_back(-5);
    nums.push_back(2);
    nums.push_back(-1);

    NumArray numArray(nums);
    printf("%d\n",numArray.sumRange(0, 1));
    printf("%d\n",numArray.sumRange(1, 2));
    return 0;
}

顺利通过。思路很重要！！！思路很重要！！！思路很重要！！！