303. Range Sum Query - Immutable(前缀和)

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

class NumArray {
public:
    vector<int> dp;
    NumArray(vector<int>& nums) {
        dp = vector<int>(nums.size()+1,0);
        for(int i = 1; i <= nums.size();i++) {
            dp[i]= dp[i-1] + nums[i-1];
        }
    }
    
    int sumRange(int left, int right) {
        return dp[right+1] - dp[left];
    }
};

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray* obj = new NumArray(nums);
 * int param_1 = obj->sumRange(left,right);
 */

 

法一：暴力

class NumArray {

public:
    vector<int> Nums;

    NumArray(vector<int> nums) {
        Nums =nums;
    }
    
    int sumRange(int i, int j) {
        int res = 0;
        for(int k = i;k<=j;k++)
            res+=Nums[k];
        return res;
    }
};

 

 

 

法二：

sumRange(i,j)=sum[j+1]−sum[i]

 

 

class NumArray {

public:
    vector<int> Nums;
    vector<int> Sums;


    NumArray(vector<int> nums) {
        Nums =nums;
        Sums = vector<int>(nums.size()+1,0);
        for(int i = 0;i<nums.size();i++)
            Sums[i+1]=Sums[i]+nums[i];
    }
    
    int sumRange(int i, int j) {
        int res = 0;
        res = Sums[j+1]-Sums[i];
        return res;
    }
};