2020-04-12

用前序和中序遍历构建二叉树

/**

* Definition for binary tree

* struct TreeNode {

*    int val;

*    TreeNode *left;

*    TreeNode *right;

*    TreeNode(int x) : val(x), left(NULL), right(NULL) {}

* };

*/

class Solution {

public:

    TreeNode* reConstructBinaryTree(vector pre,vector vin) {

        TreeNode* tree;

        tree = GetSubTree(pre,vin,0,pre.size()-1,0,vin.size()-1);

        return tree;

    }

    TreeNode* GetSubTree(vector&pre, vector&vin, int pre_l,int pre_r,int in_l,int in_r)

    {

        if(pre_l>pre_r||in_l>in_r)

        { 

            return NULL;

        }

        //中序遍历中的父节

        int mid = in_l;

        while(mid<=in_r&&vin[mid]!=pre[pre_l]){mid++;}

        //得到左子子树的长度

        int left_num = mid-in_l;

        TreeNode* T = new TreeNode(pre[pre_l]);

        T->left = GetSubTree(pre,vin,pre_l+1, left_num+pre_l,in_l,mid-1);

        T->right = GetSubTree(pre,vin,left_num+pre_l+1,pre_r,mid+1,in_r);

        return T;

    }

};