力扣每日一题-区域和检索 - 数组可修改

307. 区域和检索 - 数组可修改 - 力扣（LeetCode）

看题面单点修改加区间查询 == 一眼线段树 >> 由于没有复杂修改操作 - > 简化为树状数组求解

int init = []()
{
    cin.tie(0) -> sync_with_stdio(false);
    return 0;
}();

class NumArray {
public:
    NumArray(vector<int>& nums) : sum(nums.size() + 1, 0), num(nums), n(nums.size()) 
    {
        for(int i = 0;i < n;i++)
        add(i + 1,nums[i]);
    }

    int lowbit(int x)
    {
        return x & -x;
    }

    void add(int pos, int val)
    {
        for(; pos <= n; pos += lowbit(pos))
        sum[pos] += val;
    }

    void update(int index, int val) 
    {
        add(index + 1, val - num[index]);
        num[index] = val;
    }
    
    int ask(int x)
    {
        int ans = 0;
        for(; x > 0; x -= lowbit(x))
        ans += sum[x];
        return ans;
    }

    int sumRange(int left, int right) 
    {
        return ask(right + 1) - ask(left);
    }
private:
vector<int>sum;
vector<int>&num;
int n;
};

/**
 * 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);
 */

还有一个有意思的解法

int init = []()
{
    cin.tie(0) -> sync_with_stdio(false);
    return 0;
}();

class NumArray 
{
public:
    NumArray(vector<int>& nums) 
    {
        n = nums.size();
        v.resize(n << 1);
        for(int i = n; i < (n << 1); i++)
        v[i] = nums[i - n];
        for(int i = n - 1; i > 0; i--)
        v[i] = v[i << 1] + v[(i << 1) + 1];
    }
    
    void update(int index, int val) 
    {
        int i = n + index;
        int diff = val - v[i];
        v[i] = val;
        while(i > 1)
        {
            v[i >> 1] += diff;
            i >>= 1;
        }
    }
    
    int sumRange(int left, int right) 
    {
        left += n;right += n; 
        int sum = 0;
        while(left <= right)
        {
            if(left & 1){ sum += v[left]; left++; }
            if(!(right & 1)){ sum += v[right]; right--; }  
            left >>= 1;right >>= 1;
        }
        return sum;
    }
private:
    vector<int> v;
    int n;
};