feat: construct binary tree from preorder and inorder traversal

Author: minji-go

construct-binary-tree-from-preorder-and-inorder-traversal/minji-go.java

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+/*
+    Problem: https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
+    Description: Given two integer arrays preorder and inorder, construct and return the binary tree.
+    Concept: Array, Hash Table, Divide and Conquer, Tree, Binary Tree
+    Time Complexity: O(NM), Runtime 2ms
+    Space Complexity: O(N), Memory 45.02MB
+*/
+import java.util.HashMap;
+import java.util.Map;
+
+class Solution {
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        boolean isLeft = true;
+        Map<Integer, TreeNode> parents = new HashMap<>();
+        TreeNode rootNode = null, parentNode = null;
+
+        for (int pidx=0, iidx=0; pidx<preorder.length; pidx++) {
+            int pval = preorder[pidx];
+            int ival = inorder[iidx];
+
+            if(pidx==0) {
+                rootNode = parentNode = new TreeNode(pval);
+            } else if (isLeft) {
+                parents.put(pval, parentNode);
+                parentNode = parentNode.left = new TreeNode(pval);
+            } else {
+                isLeft = true;
+                parents.put(pval, parentNode);
+                parentNode = parentNode.right = new TreeNode(pval);
+            }
+
+            if(pval==ival) {
+                isLeft = false;
+                TreeNode targetNode = parentNode;
+                while (iidx<inorder.length-1 && parents.get(parentNode.val)!=null) {
+                    if(parentNode.val == inorder[iidx+1]){
+                        iidx++;
+                        targetNode = parentNode;
+                    }
+                    parentNode = parents.get(parentNode.val);
+                }
+                if(iidx<inorder.length-1 && parentNode.val != inorder[iidx+1]){
+                    iidx++;
+                    parentNode = targetNode;
+                } else iidx = iidx+2;
+            }
+        }
+
+        return rootNode;
+    }
+}
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */