Solution for "Binary Tree Vertical Order Traversal"

Author: Thuy Trinh

src/main/java/leetcode/btverticalordertraversal/Solution.kt

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+package leetcode.btverticalordertraversal
+
+import leetcode.binarytreeboundary.TreeNode
+import java.util.*
+
+/**
+ * https://leetcode.com/problems/binary-tree-vertical-order-traversal/description/
+ */
+class Solution {
+  fun verticalOrder(root: TreeNode?): List<List<Int>> = when {
+    root != null -> {
+      val m = root.travelVertically()
+      m.keys.sorted()
+          .map { m[it] ?: mutableListOf() }
+    }
+    else -> emptyList()
+  }
+
+  private fun TreeNode.travelVertically(): MutableMap<Int, MutableList<Int>> {
+    val map = mutableMapOf<Int, MutableList<Int>>()
+    val xCache = mutableMapOf(this to 0)
+    val nodes = LinkedList<TreeNode>()
+    nodes.offer(this)
+    while (nodes.isNotEmpty()) {
+      val node = nodes.poll()
+      val x = xCache[node] ?: 0
+      map.getOrPut(x) { mutableListOf() }.add(node.`val`)
+      node.left?.let {
+        nodes.offer(it)
+        xCache[it] = x - 1
+      }
+      node.right?.let {
+        nodes.offer(it)
+        xCache[it] = x + 1
+      }
+    }
+    return map
+  }
+}

src/test/java/leetcode/btverticalordertraversal/SolutionTest.kt

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+package leetcode.btverticalordertraversal
+
+import leetcode.binarytreeboundary.node
+import org.amshove.kluent.shouldEqual
+import org.junit.Test
+
+class SolutionTest {
+  @Test
+  fun test() {
+    Solution().verticalOrder(node(3) {
+      left = node(9)
+      right = node(20) {
+        left = node(15)
+        right = node(7)
+      }
+    }).shouldEqual(listOf(
+        listOf(9),
+        listOf(3, 15),
+        listOf(20),
+        listOf(7)
+    ))
+
+    Solution().verticalOrder(node(3) {
+      left = node(9) {
+        left = node(4)
+        right = node(0)
+      }
+      right = node(8) {
+        left = node(1)
+        right = node(7)
+      }
+    }).shouldEqual(listOf(
+        listOf(4),
+        listOf(9),
+        listOf(3, 0, 1),
+        listOf(8),
+        listOf(7)
+    ))
+  }
+}