Do the same for right to left
We can do the exact same thing from right to left. This will be accumulated in a new array named right[].
So finally, we get the two arrays left[] and right[] that we can use to get the maximum water level at each column. See the code to understand how we got these two.
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console.log(totalRain(arr));function totalRain(heights) { if (heights === null || heights.length <= 2) return 0;​ let N = heights.length; let max = 0; let left = Array(N).fill(0); let right = Array(N).fill(0);​ // scan from left to right max = heights[0]; left[0] = max; for (let i = 1; i < N; i++) { if (heights[i] > max) { max = heights[i]; } left[i] = max; } // scan from right to left max = heights[N - 1]; right[N - 1] = max; for (let i = N - 2; i >= 0; i--) { if (heights[i] > max) { max = heights[i]; } right[i] = max; } return right;}OUTPUT
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