Construct Binary Tree From Preorder And Inorder Traversal

construct-binary-tree-from-preorder-and-inorder-traversal/sunjae95.js

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+/**
+ * @description
+ * time complexity: O(n^2)
+ * space complexity: O(n)
+ *
+ * brainstorming:
+ * stack, Drawing a graph
+ *
+ * strategy:
+ * discover the rules
+ * leftStack = left create , rightStack = right create
+ */
+var buildTree = function (preorder, inorder) {
+  let answer = null;
+  let pointer = 0;
+
+  const leftStack = [];
+  const rightStack = [];
+
+  preorder.forEach((val, i) => {
+    const node = new TreeNode(val);
+
+    if (i === 0) answer = node;
+
+    const leftLen = leftStack.length;
+    const rightLen = rightStack.length;
+
+    if (leftLen && rightLen) {
+      if (leftStack[leftLen - 1].left) rightStack[rightLen - 1].right = node;
+      else leftStack[leftLen - 1].left = node;
+    }
+    if (leftLen && !rightLen) leftStack[leftLen - 1].left = node;
+    if (!leftLen && rightLen) rightStack[rightLen - 1].right = node;
+
+    leftStack.push(node);
+
+    while (leftStack.length && pointer < inorder.length) {
+      if (leftStack[leftStack.length - 1].val !== inorder[pointer]) break;
+      rightStack.push(leftStack.pop());
+      pointer++;
+    }
+  });
+
+  return answer;
+};