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42: Trapping Rain Water
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‎README.md‎

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@@ -10,6 +10,7 @@ My solutions to LeetCode problems in Kotlin.
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| [5](https://leetcode.com/problems/longest-palindromic-substring/) | [Longest Palindromic Substring](src/main/kotlin/com/schmoczer/leetcode/_0005/LongestPalindromicSubstring.kt) | Medium |
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| [8](https://leetcode.com/problems/string-to-integer-atoi/) | [String to Integer (atoi)](src/main/kotlin/com/schmoczer/leetcode/_0008/StringToInteger.kt) | Medium |
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| [20](https://leetcode.com/problems/valid-parentheses/) | [Valid Parentheses](src/main/kotlin/com/schmoczer/leetcode/_0020/ValidParentheses.kt) | Easy |
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| [42](https://leetcode.com/problems/trapping-rain-water/) | [Trapping Rain Water](src/main/kotlin/com/schmoczer/leetcode/_0042/TrappingRainWater.kt) | Hard |
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| [49](https://leetcode.com/problems/group-anagrams/) | [Group Anagrams](src/main/kotlin/com/schmoczer/leetcode/_0049/GroupAnagrams.kt) | Medium |
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| [125](https://leetcode.com/problems/valid-palindrome/) | [Valid Palindrome](src/main/kotlin/com/schmoczer/leetcode/_0125/ValidPalindrome.kt) | Easy |
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| [151](https://leetcode.com/problems/reverse-words-in-a-string/) | [Reverse Words in a String](src/main/kotlin/com/schmoczer/leetcode/_0151/ReverseWordsInString.kt) | Medium |
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# Trapping Rain Water
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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
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it can trap after raining.
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Example 1:
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> Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
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>
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> Output: 6
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Example 2:
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> Input: height = [4,2,0,3,2,5]
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>
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> Output: 9
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Constraints:
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- `n == height.length`
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- `1 <= n <= 2 * 10^4`
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- `0 <= height[i] <= 10^5`
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package com.schmoczer.leetcode._0042
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import kotlin.math.max
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import kotlin.math.min
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class TrappingRainWater {
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// Approach 1: bruteforce
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fun trap1(height: IntArray): Int {
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if (height.size < 3) {
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return 0
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}
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var water = 0
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var leftHeight = height.first()
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var rightHeight = height.copyOfRange(2, height.size).max()
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for (i in 1 until height.size - 1) {
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val currentHeight = height[i]
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height[i - 1].let {
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if (it > leftHeight) {
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leftHeight = it
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}
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}
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if (currentHeight == rightHeight) {
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rightHeight = height.copyOfRange(i + 1, height.size).max()
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}
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val lowest = min(leftHeight, rightHeight)
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if (lowest > currentHeight) {
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water += lowest - currentHeight
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}
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}
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return water
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}
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// Approach 2: 2 pointers, Runtime 1ms Beats 100.00%
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fun trap(height: IntArray): Int {
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if (height.size < 3) {
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return 0
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}
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var water = 0
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var leftIndex = 0
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var rightIndex = height.size - 1
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var leftHeight = 0
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var rightHeight = 0
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while (leftIndex < rightIndex) {
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if (height[leftIndex] < height[rightIndex]) {
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leftHeight = max(leftHeight, height[leftIndex])
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water += leftHeight - height[leftIndex]
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leftIndex++
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} else {
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rightHeight = max(rightHeight, height[rightIndex])
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water += rightHeight - height[rightIndex]
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rightIndex--
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}
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}
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return water
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}
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}
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package com.schmoczer.leetcode._0042
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import org.junit.jupiter.api.BeforeEach
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import kotlin.test.Test
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import kotlin.test.assertEquals
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class TrappingRainWaterTests {
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private lateinit var sut: TrappingRainWater
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@BeforeEach
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fun setUp() {
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sut = TrappingRainWater()
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}
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@Test
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fun `0,1,0,2,1,0,1,3,2,1,2,1 traps 6 water`() {
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val input = intArrayOf(0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1)
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assertEquals(6, sut.trap(input))
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}
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@Test
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fun `4,2,0,3,2,5 traps 9 water`() {
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val input = intArrayOf(4, 2, 0, 3, 2, 5)
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assertEquals(9, sut.trap(input))
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}
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}

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