// File: build_tree.go // Created Time: 2023-07-20 // Author: hongyun-robot (1836017030@qq.com) package chapter_divide_and_conquer import . "github.com/krahets/hello-algo/pkg" /* Build binary tree: Divide and conquer */ func dfsBuildTree(preorder []int, inorderMap map[int]int, i, l, r int) *TreeNode { // Terminate when subtree interval is empty if r-l < 0 { return nil } // Initialize root node root := NewTreeNode(preorder[i]) // Query m to divide left and right subtrees m := inorderMap[preorder[i]] // Subproblem: build left subtree root.Left = dfsBuildTree(preorder, inorderMap, i+1, l, m-1) // Subproblem: build right subtree root.Right = dfsBuildTree(preorder, inorderMap, i+1+m-l, m+1, r) // Return root node return root } /* Build binary tree */ func buildTree(preorder, inorder []int) *TreeNode { // Initialize hash table, storing in-order elements to indices mapping inorderMap := make(map[int]int, len(inorder)) for i := 0; i < len(inorder); i++ { inorderMap[inorder[i]] = i } root := dfsBuildTree(preorder, inorderMap, 0, 0, len(inorder)-1) return root }