Construct Binary Tree from Preorder and Inorder Traversal

Construct Binary Tree from Preorder and Inorder Traversal.png

解題思路 ：

利用 preorder 的頭找到 root 的值 再用 root 值去 inorder 找到 root 在 inorder 中的 index 接著用 inorder 的頭到 index -1 的範圍當作左子數的範圍 index + 1 到 index 尾的範圍當右邊子樹的範圍繼續 recursive

C++ code :

 /**


Definition of TreeNode:
class TreeNode {
public:
int val;

TreeNode *left, *right;

TreeNode(int val) {

    this->val = val;

    this->left = this->right = NULL;

}

}
 */

class Solution {
/**
 *@param preorder : A list of integers that preorder traversal of a tree
 *@param inorder : A list of integers that inorder traversal of a tree
 *@return : Root of a tree
 */

public:
int getIndex(vector &v, int value)
{
    for(int i = 0; i < v.size(); i++)
    {
        if(v[i] == value) return i;
    }
    return -1;
}

TreeNode *construct(vector &pre, int Pstart, int Pend, vector &in, int Istart, int Iend)
{
    if (Pstart > Pend) return nullptr;
    int root_val = pre[Pstart];
    int index = getIndex(in, root_val);
    TreeNode *root = new TreeNode(root_val);
    root->left = construct(pre, Pstart + 1, Pstart + (index - Istart), in, Istart, index - 1);
    root->right = construct(pre, Pstart + (index - Istart) + 1, Pend, in, index + 1, Iend);
    return root;
}

TreeNode *buildTree(vector &preorder, vector &inorder) {
    // write your code here
    int Pend = preorder.size() - 1;
    int Iend = inorder.size() - 1;
    return construct(preorder, 0, Pend, inorder, 0, Iend);
}

};