LC.P303[区域和检索-数组不可变]

方法一：前缀和

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class NumArray {

    int[] sum;

    public NumArray(int[] nums) {
        int n = nums.length;
        sum = new int[n + 1];
        for (int i = 1; i <= n; ++i) {
            sum[i] = sum[i - 1] + nums[i - 1];
        }
    }
    
    public int sumRange(int left, int right) {
        return sum[right + 1] - sum[left];
    }
}

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(left,right);
 */
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$