Published Dec 18, 2021
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Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.
Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6
Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.
Input: height = [4,2,0,3,2,5]
Output: 9
- n == height.length
- 1 <= n <= 2 * 104
- 0 <= height[i] <= 105
function trap(height: number[]): number {
let l = 0, r = height.length - 1
// we store leftMax as first value and rightMax as last value
let leftMax = height[l], rightMax = height[r]
let res = 0;
// we have to pointers moving to the mid
while (l < r) {
// decide which pointer to move
if(leftMax < rightMax) {
l++;
// update max value
leftMax = Math.max(leftMax, height[l])
// update res
res += leftMax - height[l];
} else {
r--;
rightMax = Math.max(rightMax, height[r])
res += rightMax - height[r]
}
}
return res;
};