#0291

Trapping Rain Water

HardDSA40 min25 XP

Problem

Given non-negative bar heights, return how many units of water can be trapped after raining.

Why This Matters

This is a classic two-pointer proof problem where the lower boundary determines safe progress.

Function Signature

def trap_rain_water(height):

Examples

Example 1

Inputheight = [0,1,0,2,1,0,1,3,2,1,2,1]

Output6

The dips between taller bars hold 6 total units of water.

Constraints

Return the exact requested value.
Handle edge cases cleanly.
Use the intended DSA pattern when brute force would scale poorly.

CodePython

def trap_rain_water(height):
    """
    Input example: height = [0,1,0,2,1,0,1,3,2,1,2,1]
    Return: 6
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

def trap_rain_water(height):
    """
    Input example: height = [0,1,0,2,1,0,1,3,2,1,2,1]
    Return: 6
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

Visible browser tests run here when available.

Testcases2 visible / 3 hidden categories

Classic terrain

Input[0,1,0,2,1,0,1,3,2,1,2,1]

Expected6

Water accumulates in multiple valleys.

No trap

Input[1,2,3]

Expected0

There is no right wall for earlier bars.

Hidden Test Categories

empty inputflat barssingle valley