complete trapping rain water

Dynamic-Programming/TrappingRainWater.js

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+/**
+ * @param {number[]} height
+ * @return {number}
+ */
+
+/* 42. Trapping Rain Water
+https://leetcode.com/problems/trapping-rain-water/
+
+Helpful animation of this prompt: https://youtu.be/HmBbcDiJapY?t=51
+
+Given n non-negative integers representing an elevation map where
+the width of each bar is 1, compute how much water it is able to trap
+after raining.
+
+VIEW ELEVATION MAP ON LEETCODE
+
+Example:
+
+Input:            [0,1,0,2,1,0,1,3,2,1,2,1]
+Output: 6
+
+Plan:
+iterate through and find left maxes
+iterate through and find right maxes
+create minheight and assign it to the min(leftmax, rightmax)
+if current height(element) < minheight
+  push minheight - height into water array
+else
+  push 0 onto water array
+
+sum up water array and return
+
+left maxes =      [0,0,1,1,2,2,2,2,3,3,3,3]
+right maxes =     [3,3,3,3,3,3,3,2,2,2,1,0]
+water contained = [0,0,1,0,1,2,1,0,0,1,0,0] -> sum = 6
+*/
+
+function trap (heights) {
+  const maxes = new Array(heights.length).fill(0)
+
+  let leftMax = 0
+  for (let i = 0; i < heights.length; i++) {
+    const height = heights[i]
+    maxes[i] = leftMax
+    leftMax = Math.max(leftMax, height)
+  }
+
+  let rightMax = 0
+  for (let i = heights.length - 1; i >= 0; i -= 1) {
+    const height = heights[i]
+    const minHeight = Math.min(rightMax, maxes[i])
+
+    if (height < minHeight) {
+      maxes[i] = minHeight - height
+    } else {
+      maxes[i] = 0
+    }
+    rightMax = Math.max(rightMax, height)
+  }
+  return maxes.reduce((a, b) => a + b, 0)
+}
+
+console.log(trap([0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1])) // -> 6