Merge pull request soapyigu#342 from soapyigu/Tree

Add a solution to Step-By-Step Directions From a Binary Tree Node to …

Tree/StepByStepDirectionsBinaryTreeNode.swift

@@ -0,0 +1,67 @@
+/**
+ * Question Link: https://leetcode.com/problems/step-by-step-directions-from-a-binary-tree-node-to-another/
+ * Primary idea: The shortest path should pass LCA. Find paths for two nodes and remove their longgest common prefix.
+ * Time Complexity: O(n), Space Complexity: O(n)
+ * 
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.left = nil
+ *         self.right = nil
+ *     }
+ * }
+ */
+
+class StepByStepDirectionsBinaryTreeNode {
+    func getDirections(_ root: TreeNode?, _ startValue: Int, _ destValue: Int) -> String {
+        guard let root = root else {
+            return ""
+        }
+
+        let startPath = getPath(root, startValue)
+        let destPath = getPath(root, destValue)
+        let len = longestCommonPrefixLen(startPath, destPath)
+
+        return String(repeating: "U", count: startPath.count - len) + destPath.dropFirst(len)
+    }
+
+    private func longestCommonPrefixLen(_ s: String, _ d: String) -> Int {
+        var i = 0
+        let s = Array(s), d = Array(d)
+
+        while i < min(s.count, d.count) {
+            if s[i] != d[i] {
+                break
+            }
+
+            i += 1
+        }
+
+        return i
+    }
+
+    private func getPath(_ parent: TreeNode, _ val: Int) -> String {       
+        var queue = [(parent, "")]
+
+        while !queue.isEmpty {
+            let current = queue.removeFirst()
+
+            if current.0.val == val {
+                return current.1
+            }
+
+            if let left = current.0.left {
+                queue.append((left, current.1 + "L"))
+            }
+            if let right = current.0.right {
+                queue.append((right, current.1 + "R"))
+            }
+        }
+
+        return ""
+    }
+}