106

Author: tbswang

106.从中序与后序遍历序列构造二叉树.cpp

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+/*
+ * @lc app=leetcode.cn id=106 lang=cpp
+ *
+ * [106] 从中序与后序遍历序列构造二叉树
+ */
+
+#include <iostream>
+#include <vector>
+#include <queue>
+using namespace std;
+struct TreeNode
+{
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode() : val(0), left(nullptr), right(nullptr) {}
+    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+};
+
+// @lc code=start
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution
+{
+public:
+    TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder)
+    {
+        return build(inorder, 0, inorder.size() - 1, postorder, 0, postorder.size() - 1);
+    }
+    TreeNode *build(vector<int> &inorder, int inStart, int inEnd, vector<int> &postorder, int postStart, int postEnd)
+    {
+        if (inStart > inEnd)
+            return NULL;
+        auto root = new TreeNode(postorder[postEnd]);
+        // 从中序遍历找到根
+        int idx = inStart;
+        for (; idx <= inEnd; idx++)
+        {
+            if (inorder[idx] == postorder[postEnd])
+            {
+                break;
+            }
+        }
+        int leftSize = idx - inStart;
+        // 左子树
+        root->left = build(inorder, inStart, idx - 1, postorder, postStart, postStart + leftSize - 1);
+        root->right = build(inorder, idx + 1, inEnd, postorder, postStart + leftSize, postEnd - 1);
+        return root;
+    }
+};
+// @lc code=end
+
+int main()
+{
+    Solution s;
+    vector<int> pre = {9, 3, 15, 20, 7};
+    vector<int> in = {9, 15, 7, 20, 3};
+    TreeNode *root = s.buildTree(pre, in);
+    // 层序遍历
+    queue<TreeNode *> q;
+    q.push(root);
+    // auto a = new TreeNode(9);
+    // cout << a->val;
+    // cout << a->val;
+    cout << '[';
+    while (!q.empty())
+    {
+        TreeNode *now = q.front();
+        cout << now->val;
+        q.pop();
+        if (now->left != nullptr)
+        {
+            q.push(now->left);
+        }
+        else
+        {
+            cout << "null";
+        }
+        if (now->right != nullptr)
+        {
+            q.push(now->right);
+        }
+        else
+        {
+            cout << "null";
+        }
+        cout << ",";
+    }
+    cout << "]";
+}