[Hyun] Week 3 Solution Explanation

climbing-stairs/WhiteHyun.swift

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+//  
+//  70. Climbing Stairs
+//  https://leetcode.com/problems/climbing-stairs/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/12.
+//  
+
+final class Solution {
+  func climbStairs(_ n: Int) -> Int {
+    var prevWays = 1
+    var prevPrevWays = 1
+    for _ in stride(from: 2, through: n, by: 1) {
+      (prevWays, prevPrevWays) = (prevPrevWays, prevWays + prevPrevWays)
+    }
+    return prevPrevWays
+  }
+}

maximum-depth-of-binary-tree/WhiteHyun.swift

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+//  
+//  104. Maximum Depth of Binary Tree
+//  https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/12.
+//  
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init() { self.val = 0; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+final class Solution {
+  func maxDepth(_ node: TreeNode?) -> Int {
+    guard let node else { return 0 }
+
+    return max(maxDepth(node.left), maxDepth(node.right)) + 1
+  }
+}

meeting-rooms/WhiteHyun.swift

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+//  
+//  252. Meeting Rooms
+//  https://leetcode.com/problems/meeting-rooms/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/12.
+//  
+
+
+/*
+Definition of Interval:
+class Interval {
+    var start: Int
+    var end: Int
+    init() { start = 0; end = 0; }
+    init(_ a: Int, _ b: Int) { start = a; end = b }
+}
+ */
+final class Solution {
+  func canAttendMeetings(_ intervals: [Interval]) -> Bool {
+    let sortedIntervals = intervals.sorted { lhs, rhs in
+      lhs.start < rhs.start
+    }
+
+    for i in sortedIntervals.indices.dropFirst() where sortedIntervals[i - 1].end > sortedIntervals[i].start {
+      return false
+    }
+
+    return true
+  }
+}

same-tree/WhiteHyun.swift

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+//  
+//  100. Same Tree
+//  https://leetcode.com/problems/same-tree/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/12.
+//  
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init() { self.val = 0; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+extension TreeNode: Equatable {
+  public static func == (lhs: TreeNode, rhs: TreeNode) -> Bool {
+    lhs.val == rhs.val && lhs.left == rhs.left && lhs.right == rhs.right
+  }
+}
+
+final class Solution {
+  func isSameTree(_ p: TreeNode?, _ q: TreeNode?) -> Bool {
+    p == q  
+  }
+}

subtree-of-another-tree/WhiteHyun.swift

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+//  
+//  572. Subtree of Another Tree
+//  https://leetcode.com/problems/subtree-of-another-tree/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/12.
+//  
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init() { self.val = 0; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+extension TreeNode: Equatable {
+  public static func == (lhs: TreeNode, rhs: TreeNode) -> Bool {
+    lhs.val == rhs.val && lhs.left == rhs.left && lhs.right == rhs.right
+  }
+}
+
+final class Solution {
+  func isSubtree(_ node: TreeNode?, _ subRoot: TreeNode?) -> Bool {
+    if node == subRoot { return true }
+    else if node?.left != nil, isSubtree(node?.left, subRoot) { return true }
+    else if node?.right != nil, isSubtree(node?.right, subRoot) { return true }
+    return false
+  }
+}