diff --git a/Algorithms/1028. Recover a Tree From Preorder Traversal/1028. Recover a Tree From Preorder Traversal.go b/Algorithms/1028. Recover a Tree From Preorder Traversal/1028. Recover a Tree From Preorder Traversal.go new file mode 100644 index 00000000..546eb899 --- /dev/null +++ b/Algorithms/1028. Recover a Tree From Preorder Traversal/1028. Recover a Tree From Preorder Traversal.go @@ -0,0 +1,71 @@ +package leetcode + +import ( + "strconv" +) + +/** + * Definition for a binary tree node. + * type TreeNode struct { + * Val int + * Left *TreeNode + * Right *TreeNode + * } + */ +func recoverFromPreorder(S string) *TreeNode { + if len(S) == 0 { + return &TreeNode{} + } + root, index, level := &TreeNode{}, 0, 0 + cur := root + dfsBuildPreorderTree(S, &index, &level, cur) + return root.Right +} + +func dfsBuildPreorderTree(S string, index, level *int, cur *TreeNode) (newIndex *int) { + if *index == len(S) { + return index + } + if *index == 0 && *level == 0 { + i := 0 + for i = *index; i < len(S); i++ { + if !isDigital(S[i]) { + break + } + } + num, _ := strconv.Atoi(S[*index:i]) + tmp := &TreeNode{Val: num, Left: nil, Right: nil} + cur.Right = tmp + nLevel := *level + 1 + index = dfsBuildPreorderTree(S, &i, &nLevel, tmp) + index = dfsBuildPreorderTree(S, index, &nLevel, tmp) + } + i := 0 + for i = *index; i < len(S); i++ { + if isDigital(S[i]) { + break + } + } + if *level == i-*index { + j := 0 + for j = i; j < len(S); j++ { + if !isDigital(S[j]) { + break + } + } + num, _ := strconv.Atoi(S[i:j]) + tmp := &TreeNode{Val: num, Left: nil, Right: nil} + if cur.Left == nil { + cur.Left = tmp + nLevel := *level + 1 + index = dfsBuildPreorderTree(S, &j, &nLevel, tmp) + index = dfsBuildPreorderTree(S, index, level, cur) + } else if cur.Right == nil { + cur.Right = tmp + nLevel := *level + 1 + index = dfsBuildPreorderTree(S, &j, &nLevel, tmp) + index = dfsBuildPreorderTree(S, index, level, cur) + } + } + return index +} diff --git a/Algorithms/1028. Recover a Tree From Preorder Traversal/1028. Recover a Tree From Preorder Traversal_test.go b/Algorithms/1028. Recover a Tree From Preorder Traversal/1028. Recover a Tree From Preorder Traversal_test.go new file mode 100644 index 00000000..38a63a32 --- /dev/null +++ b/Algorithms/1028. Recover a Tree From Preorder Traversal/1028. Recover a Tree From Preorder Traversal_test.go @@ -0,0 +1,51 @@ +package leetcode + +import ( + "fmt" + "testing" +) + +type question1028 struct { + para1028 + ans1028 +} + +// para 是参数 +// one 代表第一个参数 +type para1028 struct { + one string +} + +// ans 是答案 +// one 代表第一个答案 +type ans1028 struct { + one []int +} + +func Test_Problem1028(t *testing.T) { + + qs := []question1028{ + question1028{ + para1028{"1-2--3--4-5--6--7"}, + ans1028{[]int{1, 2, 5, 3, 4, 6, 7}}, + }, + + question1028{ + para1028{"1-2--3---4-5--6---7"}, + ans1028{[]int{1, 2, 5, 3, NULL, 6, NULL, 4, NULL, 7}}, + }, + + question1028{ + para1028{"1-401--349---90--88"}, + ans1028{[]int{1, 401, NULL, 349, 88, 90}}, + }, + } + + fmt.Printf("------------------------Leetcode Problem 1028------------------------\n") + + for _, q := range qs { + _, p := q.ans1028, q.para1028 + fmt.Printf("【input】:%v 【output】:%v\n", p, Tree2ints(recoverFromPreorder(p.one))) + } + fmt.Printf("\n\n\n") +} diff --git a/Algorithms/1028. Recover a Tree From Preorder Traversal/README.md b/Algorithms/1028. Recover a Tree From Preorder Traversal/README.md new file mode 100755 index 00000000..afd39d11 --- /dev/null +++ b/Algorithms/1028. Recover a Tree From Preorder Traversal/README.md @@ -0,0 +1,56 @@ +# [1028. Recover a Tree From Preorder Traversal](https://leetcode.com/problems/recover-a-tree-from-preorder-traversal/) + + +## 题目: + +We run a preorder depth first search on the `root` of a binary tree. + +At each node in this traversal, we output `D` dashes (where `D` is the *depth* of this node), then we output the value of this node. *(If the depth of a node is `D`, the depth of its immediate child is `D+1`. The depth of the root node is `0`.)* + +If a node has only one child, that child is guaranteed to be the left child. + +Given the output `S` of this traversal, recover the tree and return its `root`. + +**Example 1:** + +![https://assets.leetcode.com/uploads/2019/04/08/recover-a-tree-from-preorder-traversal.png](https://assets.leetcode.com/uploads/2019/04/08/recover-a-tree-from-preorder-traversal.png) + + Input: "1-2--3--4-5--6--7" + Output: [1,2,5,3,4,6,7] + +**Example 2:** + +![https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114101-pm.png](https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114101-pm.png) + + Input: "1-2--3---4-5--6---7" + Output: [1,2,5,3,null,6,null,4,null,7] + +**Example 3:** + +![https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114955-pm.png](https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114955-pm.png) + + Input: "1-401--349---90--88" + Output: [1,401,null,349,88,90] + +**Note:** + +- The number of nodes in the original tree is between `1` and `1000`. +- Each node will have a value between `1` and `10^9`. + +## 题目大意 + +我们从二叉树的根节点 root 开始进行深度优先搜索。 + +在遍历中的每个节点处,我们输出 D 条短划线(其中 D 是该节点的深度),然后输出该节点的值。(如果节点的深度为 D,则其直接子节点的深度为 D + 1。根节点的深度为 0)。如果节点只有一个子节点,那么保证该子节点为左子节点。给出遍历输出 S,还原树并返回其根节点 root。 + + +提示: + +- 原始树中的节点数介于 1 和 1000 之间。 +- 每个节点的值介于 1 和 10 ^ 9 之间。 + + +## 解题思路 + +- 给出一个字符串,字符串是一个树的先根遍历的结果,其中破折号的个数代表层数。请根据这个字符串生成对应的树。 +- 这一题解题思路比较明确,用 DFS 就可以解题。边深搜字符串,边根据破折号的个数判断当前节点是否属于本层。如果不属于本层,回溯到之前的根节点,添加叶子节点以后再继续深搜。需要注意的是每次深搜时,扫描字符串的 index 需要一直保留,回溯也需要用到这个 index。