The solution can be found with a single loop. Asymptotically same as example solution.
Algorithm is variously called “caterpillar”, “inchworm” or variable sliding window.
/**
* Find smallest contiguous subarray length whose sum is *greater than or equal* to ‘S’.
* time: O(n), space: O(1)
* @default 0
* @param s - Number - the desired minimum sum
* @param arr - Array - array of positive Number
**/
const smallest_subarray_with_given_sum = function(s, arr) {
if(s === 0) return 0;
const n = arr.length;
// this is caterpillar pattern - head moves forward, tail follows
let minLen = Infinity;
let head = 0;
let tail = 0;
let currLen = 0;
let currSum = arr[0];
while(tail < n && head < n) {
currLen = head - tail + 1;
if(currSum >= s) minLen = Math.min(currLen, minLen);
if(minLen === 1) return minLen; // base: 1 element equal sum, return 1
if(currSum > s || currLen > minLen) { // pull up rear
currSum -= arr[tail];
tail += 1;
continue;
}
head += 1;
currSum += arr[head];
}
return minLen;
};