区域与检索--数组可修改

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• 参考 链接

代码

class NumArray {
    private ArrayList<Integer> sumSegmentTree;
    private int n;

    public NumArray(int[] nums) {
        n = nums.length;
        sumSegmentTree = new ArrayList<>(2 * n + 1);
        for  (int i = 0; i < n; i++) {
            sumSegmentTree.add(0);
        }

        for (int i = 0; i < n; i++) {
            sumSegmentTree.add(nums[i]);
        }

        for (int i = n - 1; i > 0; i--) {
            sumSegmentTree.set(i, sumSegmentTree.get(2 * i) + sumSegmentTree.get(2 * i + 1));
        }
    }

    public void update(int i, int val) {
        i += n;
        sumSegmentTree.set(i, val);
        while (i > 1) {
            i /= 2;
            sumSegmentTree.set(i, sumSegmentTree.get(2 * i) + sumSegmentTree.get(2 * i + 1));
        }
    }

    /** 原算法描述为range[left,right)
     * range[i,j]
     * @param i 左下标
     * @param j 右下标
     * @return 总和
     */
    public int sumRange(int i, int j) {
        int sum = 0;
        i += n;
        j += n+1;
        while (i < j) {
            if ((i & 1) == 1) {
                sum += sumSegmentTree.get(i);
                i ++;
            }
            if ((j & 1) == 1) {
                j --;
                sum += sumSegmentTree.get(j);
            }

            i >>= 1;
            j >>= 1;
        }
        return sum;
    }
}


/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * obj.update(i,val);
 * int param_2 = obj.sumRange(i,j);
 */