区域和检索 - 数组可修改 - 线段树的构造

区域和检索 - 数组可修改

线段树的构造

public class NumArray 
{
    private int[] sum; // sum[i] 表示第 i 个块的元素和
    private int size; // 块的大小
    private int[] nums;

    public NumArray(int[] nums) 
    {
        this.nums = nums;
        int n = nums.Length;
        size = (int) Math.Sqrt(n);
        sum = new int[(n + size - 1) / size]; // n/size 向上取整
        for (int i = 0; i < n; i++) 
        {
            sum[i / size] += nums[i];
        }
    }
    
    public void Update(int index, int val) 
    {
        sum[index / size] += val - nums[index];
        nums[index] = val;
    }
    
    public int SumRange(int left, int right) 
    {
        int b1 = left / size, i1 = left % size, b2 = right / size, i2 = right % size;
        if (b1 == b2) { // 区间 [left, right] 在同一块中
            int sum = 0;
            for (int j = i1; j <= i2; j++) 
            {
                sum += nums[b1 * size + j];
            }
            return sum;
        }
        int sum1 = 0;
        for (int j = i1; j < size; j++) 
        {
            sum1 += nums[b1 * size + j];
        }
        int sum2 = 0;
        for (int j = 0; j <= i2; j++) 
        {
            sum2 += nums[b2 * size + j];
        }
        int sum3 = 0;
        for (int j = b1 + 1; j < b2; j++) 
        {
            sum3 += sum[j];
        }
        return sum1 + sum2 + sum3;
    }
}

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * obj.Update(index,val);
 * int param_2 = obj.SumRange(left,right);
 */
 
详解请看这位大佬的2篇文章 通俗易懂
https://blog.csdn.net/zearot/article/details/52280189
https://blog.csdn.net/zearot/article/details/48299459