Solution.ts

function sumSubarrayMins(arr: number[]): number {
    const n: number = arr.length;
    const left: number[] = Array(n).fill(-1);
    const right: number[] = Array(n).fill(n);
    const stk: number[] = [];
    for (let i = 0; i < n; ++i) {
        while (stk.length > 0 && arr[stk.at(-1)] >= arr[i]) {
            stk.pop();
        }
        if (stk.length > 0) {
            left[i] = stk.at(-1);
        }
        stk.push(i);
    }

    stk.length = 0;
    for (let i = n - 1; ~i; --i) {
        while (stk.length > 0 && arr[stk.at(-1)] > arr[i]) {
            stk.pop();
        }
        if (stk.length > 0) {
            right[i] = stk.at(-1);
        }
        stk.push(i);
    }

    const mod: number = 1e9 + 7;
    let ans: number = 0;
    for (let i = 0; i < n; ++i) {
        ans += ((((i - left[i]) * (right[i] - i)) % mod) * arr[i]) % mod;
        ans %= mod;
    }
    return ans;
}