119. Pascal‘s Triangle II*

119. Pascal’s Triangle II*

https://leetcode.com/problems/pascals-triangle-ii/

题目描述

Given a non-negative index k where k ≤ 33, return the kth index row of the Pascal’s triangle.

Note that the row index starts from 0.

解题思路

杨辉三角形.

C++ 实现 0

20210324 更新: 使用递归更简洁:

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        if (rowIndex == 0) return {1};
        auto prev = getRow(rowIndex - 1);
        vector<int> res(prev.size() + 1, 1);
        for (int i = 1; i < res.size() - 1; ++ i) {
            res[i] = prev[i - 1] + prev[i];
        }
        return res;
    }
};

C++ 实现 1

来自 LeetCode 的 Submission, 我的实现在 C++ 实现 2 中.

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> res(rowIndex + 1, 1);
        
        int prev, cur;
        for(int i=2; i<=rowIndex; i++)
        {
            prev = res[0];
            for(int j=1; j<i; j++)
            {
                cur = res[j];
                res[j] = prev + cur;
                prev = cur;
            }
        }
        
        return res;
    }
};

C++ 实现 2

class Solution {
private:
    void generate_triangle(vector<int> &res, int time) {
        if (time >= res.size()) return;
        int n = res.size();
        int prev = res[0];
        for (int i = 1; i < time; ++i) {
            int cur = res[i];
            res[i] = prev + cur;
            prev = cur;
        }
        generate_triangle(res, time + 1);
    }
public:
    vector<int> getRow(int rowIndex) {
        if (rowIndex < 0) return {};
        vector<int> res(rowIndex + 1, 1);
        generate_triangle(res, 1);
        return res;
    }
};

C++ 实现 3

从后向前计算更为简洁.

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> result(rowIndex+1, 0);
        result[0] = 1;
        for(int i = 1; i <= rowIndex; i++) {
            for(int j = i; j >= 1; j--) {
                result[j] = result[j] + result[j-1];
            }
        }
        return result;
    }
};