Array: [1, 3, 5, 7, 9, 11]

flowchart TB
    S0["[0-5]: 36"] --> S1["[0-2]: 9"]
    S0 --> S2["[3-5]: 27"]

    S1 --> S3["[0-1]: 4"]
    S1 --> S4["[2-2]: 5"]

    S2 --> S5["[3-4]: 16"]
    S2 --> S6["[5-5]: 11"]

    S3 --> S7["[0-0]: 1"]
    S3 --> S8["[1-1]: 3"]

    S5 --> S9["[3-3]: 7"]
    S5 --> S10["[4-4]: 9"]

    style S0 fill:#7c3aed,color:#fff
    style S1 fill:#4f46e5,color:#fff
    style S2 fill:#4f46e5,color:#fff
    style S3 fill:#6366f1,color:#fff
    style S4 fill:#6366f1,color:#fff
    style S5 fill:#6366f1,color:#fff
    style S6 fill:#6366f1,color:#fff
    style S7 fill:#059669,color:#fff
    style S8 fill:#059669,color:#fff
    style S9 fill:#059669,color:#fff
    style S10 fill:#059669,color:#fff

class SegmentTree {
  constructor(arr) {
    this.n = arr.length;
    this.tree = new Array(4 * this.n); // safe size
    this._build(arr, 0, 0, this.n - 1);
  }

  _build(arr, node, start, end) {
    if (start === end) {
      this.tree[node] = arr[start];
    } else {
      const mid = Math.floor((start + end) / 2);
      const left = 2 * node + 1;
      const right = 2 * node + 2;
      this._build(arr, left, start, mid);
      this._build(arr, right, mid + 1, end);
      this.tree[node] = this.tree[left] + this.tree[right];
    }
  }

  // Query range sum [l, r]
  query(l, r) {
    return this._query(0, 0, this.n - 1, l, r);
  }

  _query(node, start, end, l, r) {
    if (r < start || l > end) return 0;          // no overlap
    if (l <= start && end <= r) return this.tree[node]; // full overlap
    const mid = Math.floor((start + end) / 2);
    const left = this._query(2 * node + 1, start, mid, l, r);
    const right = this._query(2 * node + 2, mid + 1, end, l, r);
    return left + right;
  }

  // Point update: arr[idx] = val
  update(idx, val) {
    this._update(0, 0, this.n - 1, idx, val);
  }

  _update(node, start, end, idx, val) {
    if (start === end) {
      this.tree[node] = val;
    } else {
      const mid = Math.floor((start + end) / 2);
      if (idx <= mid) {
        this._update(2 * node + 1, start, mid, idx, val);
      } else {
        this._update(2 * node + 2, mid + 1, end, idx, val);
      }
      this.tree[node] = this.tree[2 * node + 1] + this.tree[2 * node + 2];
    }
  }
}

// Usage
const st = new SegmentTree([1, 3, 5, 7, 9, 11]);
console.log(st.query(1, 4)); // 24
st.update(2, 6);             // array → [1, 3, 6, 7, 9, 11]
console.log(st.query(1, 4)); // 25