How to Fix Trapping Rain Water Errors

In this tutorial, you'll learn about How to Fix Trapping Rain Water Errors. We cover key concepts, practical examples, and best practices.

Fix trapping rain water errors when left/right max tracking incorrect or calculation uses min wrong.

Quick Fix

Wrong

def trap(h):
    n=len(h); l_max=[0]*n; r_max=[0]*n
    for i in range(1,n): l_max[i]=max(l_max[i-1],h[i-1])
    for i in range(n-2,-1,-1): r_max[i]=max(r_max[i+1],h[i+1])
    return sum(max(0,min(l_max[i],r_max[i])-h[i]) for i in range(n))

Correct but O(n) space. Two-pointer gives O(1) space.

Right

def trap(h):
    l=0; r=len(h)-1; lm=0; rm=0; ans=0
    while l<r:
        if h[l]<h[r]:
            if h[l]>=lm: lm=h[l]
            else: ans+=lm-h[l]
            l+=1
        else:
            if h[r]>=rm: rm=h[r]
            else: ans+=rm-h[r]
            r-=1
    return ans

[0,1,0,2,1,0,1,3,2,1,2,1] -> 6. O(n) time, O(1) space.

Prevention

Two-pointer from both ends. Track max seen from left and right. Water = min(l_max, r_max) - height.

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FAQ