代码随想录day15

110.平衡二叉树

    int getHeight(TreeNode* node){
        if(node == nullptr){
            return 0;
        }
        int leftHeight = getHeight(node->left);
        if(leftHeight == -1){
            return -1;
        }
        int rightHeight = getHeight(node->right);
        if(rightHeight == -1){
            return -1;
        }
        if(abs(leftHeight - rightHeight) > 1){
            return -1;
        }else{
            return 1 + max(leftHeight, rightHeight);
        }
    }
    bool isBalanced(TreeNode* root) {
        return getHeight(root) == -1 ? false : true;
    }

257.二叉树的所有路径

//回溯法

    void traverse(TreeNode* node, vector<int>& path, vector<string>& result){
        path.push_back(node->val);
        if(node->left == nullptr && node->right == nullptr){
            string p;
            int i = 0;
            for(; i < path.size()-1; i++){
                p += to_string(path[i]);
                p += "->";
            }
            p += to_string(path[i]);
            result.push_back(p);
            return;
        }
        if(node->left){
            traverse(node->left, path, result);
            path.pop_back();
        }
        if(node->right){
            traverse(node->right, path, result);
            path.pop_back();
        }
    }
    vector<string> binaryTreePaths(TreeNode* root) {
        vector<string> result;
        vector<int> path;

        traverse(root, path, result);
        return result;
    }

404.左叶子之和

    int getLeft(TreeNode* node){
        if(node == nullptr) return 0;
        int left = getLeft(node->left);
        if(node->left && !node->left->left && !node->left->right){
            left = node->left->val;
        }
        int right = getLeft(node->right);
        return left + right;
    }
    int sumOfLeftLeaves(TreeNode* root) {
        return getLeft(root);
    }

222.完全二叉树的节点个数

//掌握O(logN^2)方法

    int countNodes(TreeNode* root) {
        int result = 0;
        if(root == nullptr) return 0;
        int leftDepth = 0, rightDepth = 0;
        TreeNode* left = root->left;
        TreeNode* right = root->right;
        while(left){
            left = left->left;
            leftDepth++;
        }
        while(right){
            right = right->right;
            rightDepth++;
        }
        if(rightDepth == leftDepth){
            return (2 << leftDepth) - 1;
        }else{
            return countNodes(root->left) + countNodes(root->right) + 1;
        }
    }