0042-trapping-rain-water.ts

function trap(height: number[]): number {
    if (!height) return 0;

    let l = 0;
    let r = height.length - 1;
    let leftMax = height[l];
    let rightMax = height[r];
    let res = 0;

    while (l < r) {
        if (leftMax < rightMax) {
            l += 1;
            leftMax = Math.max(leftMax, height[l]);
            res += leftMax - height[l];
        } else {
            r -= 1;
            rightMax = Math.max(rightMax, height[r]);
            res += rightMax - height[r];
        }
    }

    return res;
}