https://leetcode.com/problems/trapping-rain-water/\n\n------------------\n\n**Follow-up:** \nWhat if now water is not coming from sky, instead we choose an index to pour x unit of water, what is the amount of water we can trap now?\n\n**Example**\n```\nInput: [2, 0, 0, 4, 0, 1]\nOutput: 4\nExplanation: if we pour 4 unit at index 2, max it can trap 4 unit of water.\n```

https://leetcode.com/problems/trapping-rain-water/\n\n------------------\n\nFollow-up: \nWhat if now water is not coming from sky, instead we choose an index to