48. leetcode 105题 由树的前序序列和中序序列构建树结构

leetcode 105题，由树的前序序列和中序序列构建树结构。详细解答参考《剑指offer》page56.

先序遍历结果的第一个节点为根节点，在中序遍历结果中找到根节点的位置。然后就可以将问题拆分，递归求解。

/**
 * 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 * buildTreeCore(vector<int>& preorder, vector<int>& inorder, int p1, int p2, int p3, int p4)
    {
        int rootValue = preorder[p1];
        TreeNode * root = new TreeNode(rootValue);
        if (p1 == p2)
        {
            if (p3 == p4 && preorder[p1] == inorder[p3])
            {
                return root;
            }

        }
        int pr = p3;
        while (pr <= p4 && inorder[pr] != rootValue)
            pr++;

        int leftLength = pr - p3;
        int p5 = p1 + leftLength;
        if (leftLength > 0)
        {
            root->left = buildTreeCore(preorder, inorder, p1 + 1, p5, p3, pr - 1);
        }
        if (leftLength < p2 - p1)
        {
            root->right = buildTreeCore(preorder, inorder, p5 + 1, p2, pr + 1, p4);
        }

        return root;
    }
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        if (preorder.size() <= 0 || inorder.size() <= 0)
        {
            return NULL;
        }
        return buildTreeCore(preorder, inorder, 0, preorder.size()-1, 0, inorder.size()-1);
    }
};