README_EN.md

118. Pascal's Triangle

中文文档

Description

Given an integer numRows, return the first numRows of Pascal's triangle.

In Pascal's triangle, each number is the sum of the two numbers directly above it as shown:

 

Example 1:

Input: numRows = 5
Output: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]

Example 2:

Input: numRows = 1
Output: [[1]]

 

Constraints:

• 1 <= numRows <= 30

Solutions

Python3

class Solution:
    def generate(self, numRows: int) -> List[List[int]]:
        if numRows == 0:
            return []
        res = []
        for i in range(numRows):
            t = [1 if j == 0 or j == i else 0 for j in range(i + 1)]
            for j in range(1, i):
                t[j] = res[i - 1][j - 1] + res[i - 1][j]
            res.append(t)
        return res

Java

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<>();
        if (numRows == 0) return res;
        for (int i = 0; i < numRows; ++i) {
            // 每一行
            List<Integer> t = new ArrayList<>();
            for (int j = 0; j < i + 1; ++j) {
                boolean firstOrLast = j == 0 || j == i;
                // 设置每一行首尾元素为1，其它元素为0
                t.add(firstOrLast ? 1 : 0);
            }
            for (int j = 1; j < i; ++j) {
                int val = res.get(i - 1).get(j - 1) + res.get(i - 1).get(j);
                t.set(j, val);
            }
            res.add(t);
        }
        return res;
    }
}

JavaScript

const generate = function (numRows) {
  let arr = [];
  for (let i = 0; i < numRows; i++) {
    let row = [];
    row[0] = 1;
    row[i] = 1;
    for (let j = 1; j < row.length - 1; j++) {
      row[j] = arr[i - 1][j - 1] + arr[i - 1][j];
    }
    arr.push(row);
  }
  return arr;
};

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