LeetCode: Construct Binary Tree from Inorder and Postorder Traversal

Given inorder and postorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void build(vector<int> &inorder, vector<int> &postorder, TreeNode* &root,
    int r, int nStart, int nEnd)
    {
        if (nStart < nEnd) return;
        
        int num = postorder[r];
        root = new TreeNode(num);
        int idx = nStart;
        while(idx >= nEnd && inorder[idx] != num)
            --idx;
        if (idx > nEnd)
            build(inorder, postorder, root->left, r-nStart+idx-1, idx-1, nEnd);
        if (idx < nStart)
            build(inorder, postorder, root->right, r-1, nStart, idx+1);
    }
    TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int nSize = inorder.size();
        if (nSize == 0) return NULL;
        TreeNode* root = NULL;
        build(inorder, postorder, root, nSize-1, nSize-1, 0);
        return root;
    }
};


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