[LeetCode]303. Range Sum Query - Immutable

[LeetCode]303. Range Sum Query - Immutable

题目描述

思路

开始的思路：存数组，将对应的区间内的数字相加
优化：
对于每一位，存入的是从第0位到当前位的和
返回的时候返回 [j] - [i - 1]即可

代码

开始的思路

class NumArray {
public:
    vector<int> nums;
    NumArray(vector<int> nums) {
        for (int p : nums)
            this->nums.push_back(p);
    }

    int sumRange(int i, int j) {
        int sum = 0;
        for (; i <= j; ++i)
            sum += nums[i];
        return sum;
    }

};

优化后

#include <iostream>
#include <vector>

using namespace std;

class NumArray {
public:
    vector<int> nums;
    NumArray(vector<int> nums) {
        if (nums.size()) {
            this->nums.push_back(nums[0]);
            for (int i = 1; i < nums.size(); ++i)
                this->nums.push_back(nums[i] + this->nums[i - 1]);
        }
    }

    int sumRange(int i, int j) {
        if (i)
            return nums[j] - nums[i - 1];
        else
            return nums[j];
    }

};

int main() {
    vector<int> nums = { -2, 0, 3, -5, 2, -1 };
    NumArray s = NumArray(nums);
    cout << s.sumRange(0, 2) << endl;
    system("pause");
}