[leetcode]404. Sum of Left Leaves

https://leetcode.com/contest/6/problems/sum-of-left-leaves/

分析:求左叶子节点的和，就是先是叶子节点，然后是左儿子，叶子节点很容易判断，左儿子需要加一个标记。

class Solution {
public:
     void dfs(TreeNode*root, bool f, int &s) {
    if(!root) return;
    if(!root->left && !root->right) {
        if(f) s += root->val;
        return;
    }
    if(root->left) {
        dfs(root->left, 1, s);
    }
    if(root->right) {
        dfs(root->right, 0, s);
    }

  }
 int sumOfLeftLeaves(TreeNode* root) {
        int res = 0;
        dfs(root, 0, res);
        return res;
    }
};