二叉树前序中序后序

前序遍历：根节点排最先，然后同级先左后右

中序遍历：先左后根最后右

后序遍历：先左后右最后根

层次遍历：按层次遍历即可

比如上图二叉树遍历结果

    前序遍历：ABCDEFGHK

    中序遍历：BDCAEHGKF

    后序遍历：DCBHKGFEA

    层次遍历：ABECFDGHK

主要分析中序遍历：

可以已知中序和前序或中序和后续求出另外一个。

领扣题：已知前序和中序求二叉树

代码拷贝自大佬博客：https://blog.youkuaiyun.com/woxiaohahaa/article/details/52744072

class Solution {
public:
    TreeNode* buildSubTree(vector<int>& preorder, vector<int>& inorder, int posPre, int posIn, int len) {
        if(0 == len) return NULL;

        int leftLen, rightLen;
        vector<int>::iterator rootPos = find(inorder.begin(), inorder.end(), preorder[posPre]);
        leftLen = rootPos - (inorder.begin() + posIn);
        rightLen = len - 1 - leftLen;
        TreeNode *root = new TreeNode(preorder[posPre]);
        root -> left = buildSubTree(preorder, inorder, posPre + 1, posIn, leftLen);
        root -> right = buildSubTree(preorder, inorder, posPre + 1 + leftLen, posIn + 1 + leftLen, rightLen);
        return root;
    }

    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        if(preorder.size() != inorder.size()) return NULL;
        if(0 == preorder.size()) return NULL; 
        return buildSubTree(preorder, inorder, 0, 0, preorder.size());
    }
};