leetcode-303. Range Sum Query - Immutable

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

Example:


思路：他给了两个函数：第一个遍历时顺带把array[i]变为前i个的和，这样计算时做个减法就行了

class NumArray {
//思路，在NumArray中求出每个索引对应的前n个的和，这样最后求值只只要做个减法就行了
vector<int> myNum;
public:
NumArray(vector<int> &nums) {
for(int i=1;i<nums.size();i++)
{
nums[i] = nums[i-1]+nums[i];
}
myNum = nums;
}

int sumRange(int i, int j) {
//保证j>=i
if(i == 0)
{
return myNum[j];
}
return myNum[j] - myNum[i-1];
}
};

// Your NumArray object will be instantiated and called as such:
// NumArray numArray(nums);
// numArray.sumRange(0, 1);
// numArray.sumRange(1, 2);