Leetcode 962. Maximum Width Ramp

Find the maximum distance j - i (i < j) such that nums[i] <= nums[j] in the array — i.e., the farthest valid "ramp". With n up to 5·10^4, brute force is too slow; this is typically solved with a monotonic decreasing stack or by sorting value-index pairs to achieve near-linear or n log n time.