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| 1 | +/* |
| 2 | + * @lc app=leetcode.cn id=105 lang=typescript |
| 3 | + * |
| 4 | + * [105] 从前序与中序遍历序列构造二叉树 |
| 5 | + * |
| 6 | + * https://leetcode.cn/problems/construct-binary-tree-from-preorder-and-inorder-traversal/description/ |
| 7 | + * |
| 8 | + * algorithms |
| 9 | + * Medium (71.32%) |
| 10 | + * Likes: 1699 |
| 11 | + * Dislikes: 0 |
| 12 | + * Total Accepted: 406.1K |
| 13 | + * Total Submissions: 569.3K |
| 14 | + * Testcase Example: '[3,9,20,15,7]\n[9,3,15,20,7]' |
| 15 | + * |
| 16 | + * 给定两个整数数组 preorder 和 inorder ,其中 preorder 是二叉树的先序遍历, inorder |
| 17 | + * 是同一棵树的中序遍历,请构造二叉树并返回其根节点。 |
| 18 | + * |
| 19 | + * |
| 20 | + * |
| 21 | + * 示例 1: |
| 22 | + * |
| 23 | + * |
| 24 | + * 输入: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7] |
| 25 | + * 输出: [3,9,20,null,null,15,7] |
| 26 | + * |
| 27 | + * |
| 28 | + * 示例 2: |
| 29 | + * |
| 30 | + * |
| 31 | + * 输入: preorder = [-1], inorder = [-1] |
| 32 | + * 输出: [-1] |
| 33 | + * |
| 34 | + * |
| 35 | + * |
| 36 | + * |
| 37 | + * 提示: |
| 38 | + * |
| 39 | + * |
| 40 | + * 1 <= preorder.length <= 3000 |
| 41 | + * inorder.length == preorder.length |
| 42 | + * -3000 <= preorder[i], inorder[i] <= 3000 |
| 43 | + * preorder 和 inorder 均 无重复 元素 |
| 44 | + * inorder 均出现在 preorder |
| 45 | + * preorder 保证 为二叉树的前序遍历序列 |
| 46 | + * inorder 保证 为二叉树的中序遍历序列 |
| 47 | + * |
| 48 | + * |
| 49 | + */ |
| 50 | + |
| 51 | +import { TreeNode } from './commons/Tree' |
| 52 | + |
| 53 | +export |
| 54 | +// @lc code=start |
| 55 | +function buildTree(preorder: number[], inorder: number[]): TreeNode | null { |
| 56 | + const inOrderIndexMapping: Record<number, number> = {} |
| 57 | + inorder.forEach((n, i) => { |
| 58 | + inOrderIndexMapping[n] = i |
| 59 | + }) |
| 60 | + |
| 61 | + function build(preBegin: number, preEnd: number, inBegin: number, inEnd: number) { |
| 62 | + if (preBegin > preEnd) return null |
| 63 | + |
| 64 | + const rootVal = preorder[preBegin] |
| 65 | + const inRoot = inOrderIndexMapping[rootVal] |
| 66 | + |
| 67 | + const leftLength = inRoot - 1 - inBegin + 1 |
| 68 | + |
| 69 | + const leftPreBegin = preBegin + 1 |
| 70 | + const leftPreEnd = leftPreBegin + leftLength - 1 |
| 71 | + const rightPreBegin = leftPreEnd + 1 |
| 72 | + const rightPreEnd = preEnd |
| 73 | + |
| 74 | + const rootNode = new TreeNode(rootVal) |
| 75 | + rootNode.left = build(leftPreBegin, leftPreEnd, inBegin, inRoot - 1) |
| 76 | + rootNode.right = build(rightPreBegin, rightPreEnd, inRoot + 1, inEnd) |
| 77 | + return rootNode |
| 78 | + } |
| 79 | + |
| 80 | + const m = preorder.length |
| 81 | + return build(0, m - 1, 0, m - 1) |
| 82 | +} |
| 83 | +// @lc code=end |
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