没有明白老师的left + index[i]的含义

/**
 * Most Significant Digit: 最重要的位，从左往右
 * @param {*} arr
 */
function msdSort(arr) {
	function sort(arr, left, right, r) {
		if (left >= right) {
			return;
		}

		let R = 3;
		let cnt = Array(R + 1).fill(0);
		let index = Array(R + 2).fill(0);
		let temp = Array(right - left + 1).fill("");
		let ACode = "A".charCodeAt(0);

		for (let i = left; i <= right; i++) {
			let code = r >= arr[i].length ? 0 : arr[i][r].charCodeAt(0) - ACode + 1;
			cnt[code]++;
		}

		for (let i = 0; i + 1 < index.length; i++) {
			index[i + 1] = index[i] + cnt[i];
		}

		for (let i = left; i <= right; i++) {
			let code = r >= arr[i].length ? 0 : arr[i][r].charCodeAt(0) - ACode + 1;
			temp[index[code]] = arr[i];
			index[code]++;
		}

		for (let i = left; i <= right; i++) {
			arr[i] = temp[i - left];
		}

		//! 方式1：重置区间索引
		/* index.fill(0);
		for (let i = 0; i + 1 < index.length; i++) {
			index[i + 1] = index[i] + cnt[i];
		} 
		for (let i = 0; i + 1 < index.length; i++) {
			let tl = index[i];
			let tr = index[i+1] - 1;
			sort(arr, tl, tr, r + 1);
		}
		*/
		//! 方式2：不重置，偏移一个区间，所以用i-1
		/* for (let i = 1; i < index.length; i++) {
			let tl = index[i - 1];
			let tr = index[i] - 1;
			sort(arr, tl, tr, r + 1);
		} */
		//! 方式2：不重置，偏移一个区间，由于排序后index[i]已变成原来的index[i+1]值,所以最后一个区间index[i]和index[i+1]重合，所以最后一个区间无需再遍历
		/* for (let i = 1; i < index.length - 1; i++) {
			let tl = index[i - 1];
			let tr = index[i] - 1;
			sort(arr, tl, tr, r + 1);
		} */
		//! 没太明白为啥是left+index[i]
		for (let i = 0; i < R; i++) {
			let tl = left + index[i];
			let tr = left + index[i + 1] - 1;
			sort(arr, tl, tr, r + 1);
		}
	}

	sort(arr, 0, arr.length - 1, 0);

	return arr;
}

let arr = ["ABC", "ACB", "AB", "BC", "BAC", "CAB", "CBA"];
msdSort(arr);
for (const str of arr) {
	console.log(str);
}

输出结果

AB
ABC
ACB
BAC
BC
CAB
CBA

感觉自己的写的方式更好理解，但是没明白老师的写法为什么也可以？