add: BinaryTreeColumns

Author: Jer3myYu

java/Trees/BinaryTreeColumns.java

@@ -0,0 +1,68 @@
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BinaryTreeColumns {
+    public class Pair {
+        TreeNode node;
+        int column;
+        public Pair(TreeNode node, int column) {
+            this.node = node;
+            this.column = column;
+        }
+    }
+
+    public List<List<Integer>> binaryTreeColumns(TreeNode root) {
+        if (root == null) {
+            return new ArrayList<>();
+        }
+        Map<Integer, List<Integer>> columnMap = new HashMap<>();
+        int leftmostColumn, rightmostColumn;
+        leftmostColumn = rightmostColumn = 0;
+        Queue<Pair> queue = new ArrayDeque<>();
+        queue.offer(new Pair(root, 0));
+        while (!queue.isEmpty()) {
+            Pair pair = queue.poll();
+            TreeNode node = pair.node;
+            int column = pair.column;
+            if (node != null) {
+                // Add the current node's value to its corresponding list in the hash 
+                // map.
+                List<Integer> columnList = columnMap.getOrDefault(column, new ArrayList<>());
+                columnList.add(node.val);
+                columnMap.put(column, columnList);
+                leftmostColumn = Math.min(leftmostColumn, column);
+                rightmostColumn = Math.max(rightmostColumn, column);
+                // Add the current node's children to the queue with their respective
+                // column ids.
+                queue.offer(new Pair(node.left, column - 1));
+                queue.offer(new Pair(node.right, column + 1));
+            }
+        }
+        // Construct the output list by collecting values from each column in the hash   
+        // map in the correct order.
+        List<List<Integer>> res = new ArrayList<>();
+        for (int i = leftmostColumn; i <= rightmostColumn; i++) {
+            List<Integer> column = columnMap.get(i);
+            res.add(column);
+        }
+        return res;
+    }
+}