左叶子之和

[题目](404. 左叶子之和)

重点

左叶子节点的概念

代码实现

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root) {
        if (root == NULL) {
            return 0;
        }
        int leftValue = 0;
        if (root->left != NULL && root->left->left == NULL && root->right->right == NULL) {
            leftValue = root->left->val;
        }
        return leftValue + sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);
    }
};

找树左下角的值

[题目](513. 找树左下角的值)

重点

使用前序遍历

代码实现

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxDepth = INT_MIN;
    int result;
    void traversal(TreeNode* root, int depth) {
        // 找到叶子节点
        if (root->left == NULL && root->right == NULL) {
            // 找最后一行的叶子节点
            if (depth > maxDepth) {
                maxDepth = depth;
                result = root->val;
            }
            return;
        }
        if (root->left) {
            depth++;
            traversal(root->left, depth);
            depth--; // 回溯
        }
        if (root->right) {
            depth++;
            traversal(root->right, depth);
            depth--; //回溯
        }
    }

    int findBottomLeftValue(TreeNode* root) {
        traversal(root, 0);
        return result;
    }
};