Improved task 42.

src/main/java/g0001_0100/s0042_trapping_rain_water/Solution.java

@@ -5,21 +5,31 @@
 
 public class Solution {
  public int trap(int[] height) {
- if (height == null || height.length <= 2) {
- return 0;
- }
  int l = 0;
  int r = height.length - 1;
- int maxL = height[l];
- int maxR = height[r];
  int res = 0;
+ int lowerWall = 0;
  while (l < r) {
- if (maxL < maxR) {
- maxL = Math.max(height[++l], maxL);
- res += maxL - height[l];
+ int lVal = height[l];
+ int rVal = height[r];
+ // If left is smaller than right ptr, make the lower wall the bigger of lVal and its
+ // current size
+ if (lVal < rVal) {
+ // If lVal has gone up, move the lowerWall upp
+ lowerWall = Math.max(lVal, lowerWall);
+ // Add the water level at current point
+ // Calculate this by taking the current value and subtracting it from the lower wall
+ // size
+ // We know that this is the lower wall because we've already determined that lVal <
+ // rVal
+ res += lowerWall - lVal;
+ // Move left ptr along
+ l++;
  } else {
- maxR = Math.max(height[--r], maxR);
- res += maxR - height[r];
+ // Do the same thing, except now we know that the lowerWall is the right side.
+ lowerWall = Math.max(rVal, lowerWall);
+ res += lowerWall - rVal;
+ r--;
  }
  }
  return res;