303. 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:

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

区间求和

C++(166ms):

class NumArray {
public:
    NumArray(vector<int> nums) {
        int sum = 0 ;
        for(int n : nums){
            sum += n ;
            res.push_back(sum) ;
        }
    }
    
    int sumRange(int i, int j) {
        if (i == 0)
            return res[j] ;
        else
            return res[j] - res[i-1] ;
    }
private:
    vector<int> res;
};

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

Java(150ms):

class NumArray {

    int[] res ;
    public NumArray(int[] nums) {
        for(int i = 1; i < nums.length ; i++){
            nums[i] += nums[i-1] ;
        }
        this.res = nums ;
    }
    
    public int sumRange(int i, int j) {
        if (i == 0)
            return res[j] ;
        else
            return res[j] - res[i-1] ;
    }
}

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