代码随想录算法训练营第十八天|● 513.找树左下角的值 ● 112. 路径总和 113.路径总和ii ● 106.从中序与后序遍历序列构造二叉树 105.从前序与中序遍历序列构造二叉树

513.找树左下角的值

题目链接：513. 找树左下角的值 - 力扣（LeetCode）

思路：利用层序遍历，找到最后一层在输出第一个节点

class Solution {
public:
    int findBottomLeftValue(TreeNode* root) {
        queue<TreeNode*>q;
        int result;
        q.push(root);
        q.push(NULL);
        TreeNode*node=root;
        result=root->val;
        while(!q.empty()){
            if(q.front()==NULL){
            q.pop();
            if(!q.empty())
                result=q.front()->val;
            else break;
            q.push(NULL);
            }
            node=q.front();
            q.pop();
            if(node->left){q.push(node->left);}
            if(node->right){q.push(node->right);}
            }
        return result;
    }
};

112路径总和  

题目链接：112. 路径总和 - 力扣（LeetCode）

class Solution {
public:
    bool traversal(TreeNode *cur,int count){
        if(!cur->right&&!cur->left&&count)return false;
        if(!cur->right&&!cur->left&&!count)return true;

        if(cur->left){
            count-=cur->left->val;
            if(traversal(cur->left,count))return true;
            count+=cur->left->val;
        }
        if(cur->right){
            count-=cur->right->val;
            if(traversal(cur->right,count))return true;
            count+=cur->right->val;
        }
        return false;
    }
    bool hasPathSum(TreeNode* root, int targetSum) {
        if(root==NULL)return false;
        return traversal(root,targetSum-root->val);
    }
};

 

106.从中序与后序遍历序列构造二叉树 

题目链接：106. 从中序与后序遍历序列构造二叉树 - 力扣（LeetCode）

本题投降

class Solution {
private:
    // 中序区间：[inorderBegin, inorderEnd)，后序区间[postorderBegin, postorderEnd)
    TreeNode* traversal (vector<int>& inorder, int inorderBegin, int inorderEnd, vector<int>& postorder, int postorderBegin, int postorderEnd) {
        if (postorderBegin == postorderEnd) return NULL;

        int rootValue = postorder[postorderEnd - 1];
        TreeNode* root = new TreeNode(rootValue);

        if (postorderEnd - postorderBegin == 1) return root;

        int delimiterIndex;
        for (delimiterIndex = inorderBegin; delimiterIndex < inorderEnd; delimiterIndex++) {
            if (inorder[delimiterIndex] == rootValue) break;
        }
        // 切割中序数组
        // 左中序区间，左闭右开[leftInorderBegin, leftInorderEnd)
        int leftInorderBegin = inorderBegin;
        int leftInorderEnd = delimiterIndex;
        // 右中序区间，左闭右开[rightInorderBegin, rightInorderEnd)
        int rightInorderBegin = delimiterIndex + 1;
        int rightInorderEnd = inorderEnd;

        // 切割后序数组
        // 左后序区间，左闭右开[leftPostorderBegin, leftPostorderEnd)
        int leftPostorderBegin =  postorderBegin;
        int leftPostorderEnd = postorderBegin + delimiterIndex - inorderBegin; // 终止位置是 需要加上 中序区间的大小size
        // 右后序区间，左闭右开[rightPostorderBegin, rightPostorderEnd)
        int rightPostorderBegin = postorderBegin + (delimiterIndex - inorderBegin);
        int rightPostorderEnd = postorderEnd - 1; // 排除最后一个元素，已经作为节点了

        root->left = traversal(inorder, leftInorderBegin, leftInorderEnd,  postorder, leftPostorderBegin, leftPostorderEnd);
        root->right = traversal(inorder, rightInorderBegin, rightInorderEnd, postorder, rightPostorderBegin, rightPostorderEnd);

        return root;
    }
public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        if (inorder.size() == 0 || postorder.size() == 0) return NULL;
        // 左闭右开的原则
        return traversal(inorder, 0, inorder.size(), postorder, 0, postorder.size());
    }
};