leetcode 968. Binary Tree Cameras

968. Binary Tree Cameras

思路：若是子节点只能覆盖到父节点、当前节点，可是父节点能够覆盖到他的父节点、子节点、当前节点，因此从叶子节点往上考虑

0表明子节点没有被覆盖

1表明子节点被覆盖，可是子节点没有camera

2表明子节点被覆盖，子节点有camera

http://www.javashuo.com/article/p-sbxbnfhk-ek.html

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int minCameraCover(TreeNode* root) {
        int sum = 0;
        if(minCameraCover(root,sum) == 0)
            sum++;
        return sum;
    }
    int minCameraCover(TreeNode* root,int& sum){
        if(!root)
            return 1;
        int left = minCameraCover(root->left,sum);
        int right = minCameraCover(root->right,sum);
        if(left == 0 || right == 0){
            sum++;
            return 2;
        }
        else if(left == 2 || right == 2)
            return 1;
        else
            return 0;
    }
};