**Leetcode 106. Construct Binary Tree from Inorder and Postorder Traversal

https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/description/

有个地方，就是，如果对于中序遍历,l->r之间的是在左子树，那么对于后续遍历 post_left到 post_left + r - l -1这个区间对应的也是左子树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        if (inorder.size() == 0) return NULL;
        return build( inorder, postorder, 0, inorder.size()-1, 0, postorder.size()-1 );
    }
    
    int getIdx(vector<int>& nodes, int target, int l, int r) {
        int sl = l;
        for (; sl <= r; sl ++) {
            if (target == nodes[sl]) {
                return sl;
            }
        }
        return -1;
    }
    
    TreeNode* build(vector<int>& inorder, vector<int>& postorder, int inl, int inr, int postl, int postr) {
        if (inr < inl || postr < postl) return NULL;
        int v = postorder[postr];
        TreeNode* root = new TreeNode(v);
        if (inl == inr) return root;
        int in_idx = getIdx(inorder, v, inl, inr);
        int post_idx =  in_idx - inl -1 + postl; 
        root->left = build( inorder, postorder, inl, in_idx-1, postl, post_idx);
        root->right = build( inorder, postorder, in_idx+1, inr, post_idx+1, postr-1 );
        return root;
    }
    
};