LN : leetcode 118 Pascal's Triangle

lc 118 Pascal's Triangle

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118 Pascal's Triangle

Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5,

Return

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

DP Accepted

这个问题就是杨辉三角，先初始化边界，再用递推式ans[i][j] = ans[i-1][j] + ans[i-1][j-1]递推即可得到答案。

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> ans(numRows);
        if (numRows <= 0)   return ans;
        for (int i = 0; i < numRows; i++)   ans[i].resize(i+1);
        for (int i = 0; i < numRows; i++)   ans[i][0] = ans[i][i] = 1;
        for (int i = 2; i < numRows; i++)
            for (int j = 1; j < i; j++)
                ans[i][j] = ans[i-1][j] + ans[i-1][j-1];
        return ans;
    }
};