第九周

62. Unique Paths

A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).
The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked 'Finish' in the diagram below).
How many possible unique paths are there?

一個m x n 的格子圖中從左上角走到右下角共有多少條路徑?

動態規劃問題，可以用組合公式。一共要走n+m-2步，在其中選取n-1步爲向右走，剩下m-1步爲向下走。共有C（n-1，m+n-2）種路徑。

class Solution {
    public:
        int uniquePaths(int m, int n) {
            int N = n + m - 2;
            int k = m - 1;             
            double res = 1;
            for (int i = 1; i <= k; i++)
                res = res * (N - k + i) / i;
            return (int)res;
        }
    };

100. Same Tree

Given two binary trees, write a function to check if they are equal or not.Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

判斷兩個二叉樹是否相同

採用遞歸的方法，判斷根的值，判斷左子樹，再判斷右子樹。

class Solution {
public:
    bool isSameTree(TreeNode* p, TreeNode* q) {
        if(p==NULL||q==NULL)
            return (p==q);
        if(p->val!=q->val)
            return false;
        if(!isSameTree(p->left,q->left))
            return false;
        if(!isSameTree(p->right,q->right))
            return false;
        return true;
    }


};