代码随想录算法训练营第十八天 | 513.找树左下角的值

513.找树左下角的值

题目链接 文章讲解 视频讲解

class Solution {
public:
    int maxDepth = INT_MIN;
    int result;
    int findBottomLeftValue(TreeNode* root) {       
        int depth = 0;
        traversal(root, depth);
        return result;
    }
    void traversal(TreeNode* node, int depth) {
        if(node->left == nullptr && node->right == nullptr) {
            if(maxDepth < depth) {
                maxDepth = depth;
                result = node->val;
            }
            return ;
        }
        if(node->left) {
            depth++;
            traversal(node->left, depth);
            depth--;
        } 
        if(node->right) {
            depth++;
            traversal(node->right, depth);
            depth--;
        }
    }
};

112.路径总和

题目链接 文章讲解 视频讲解

class Solution {
public:
    bool hasPathSum(TreeNode* root, int targetSum) {
        if(root == nullptr) return false;
        int sum = root->val;
        return traversal(root, sum, targetSum);
    }
    bool traversal(TreeNode* node, int sum, int targetSum) {
        if(node->left == nullptr && node->right == nullptr) {
            if(sum == targetSum) return true;
            return false;
        }
        if(node->left) {
            sum += node->left->val;
            if(traversal(node->left, sum, targetSum)) return true;
            sum -= node->left->val;
        } 
        if(node->right) {
            sum += node->right->val;
            if(traversal(node->right, sum, targetSum)) return true;
            sum -= node->right->val;
        }
        return false;
    }
};

113.路经总和II

题目链接

class Solution {
private:
    int total = 0;
    vector<vector<int>> ans;
    vector<int> path;
public:
    vector<vector<int>> pathSum(TreeNode* root, int targetSum) {
        if(root == nullptr) return {};
        path.push_back(root->val);
        total += root->val;
        traversal(root, targetSum);
        return ans;
    }
    void traversal(TreeNode* node, int targetSum) {
        if(node->left == nullptr && node->right == nullptr) {
            if(total == targetSum) ans.push_back(path);
            return ;
        }
        if(node->left) {
            path.push_back(node->left->val);
            total += node->left->val;
            traversal(node->left, targetSum);
            total -= node->left->val;
            path.pop_back();
        } 
        if(node->right) {
            path.push_back(node->right->val);
            total += node->right->val;
            traversal(node->right, targetSum);
            total -= node->right->val;
            path.pop_back();
        } 
        return ;
    }
};

106.从中序与后续遍历序列构造二叉树

class Solution {
public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {

        // 后序或者前序数组为空时，直接返回空
        if(postorder.size() == 0) return nullptr;
        if(postorder.size() == 1) {
            // 构造根节点
            TreeNode* root = new TreeNode(postorder[0]);    
            return root;
        }
        TreeNode * root = new TreeNode();
        root->val = postorder.back();

        // 切中序数组
        int idx = 0;
        for(; idx < inorder.size(); ++idx) {
            if(inorder[idx] == root->val)
                break;
        }
        // 左中序
        vector<int> left_inorder(inorder.begin(), inorder.begin() + idx);
        // 右中序
        vector<int> right_inorder(inorder.begin() + idx + 1, inorder.end());

        // 切后序数组
        // 左后序
        vector<int> left_postorder(postorder.begin(), postorder.begin() + left_inorder.size());
        // 右后序
        vector<int> right_postorder(postorder.begin() + left_inorder.size(), postorder.end() - 1);

        // 递归构建左子树
        root->left = buildTree(left_inorder, left_postorder);
        // 递归构建右子树
        root->right = buildTree(right_inorder, right_postorder);

        return root;
    }
};