README.md

118. 杨辉三角

English Version

题目描述

给定一个非负整数 numRows，生成杨辉三角的前 numRows 行。

在杨辉三角中，每个数是它左上方和右上方的数的和。

示例:

输入: 5
输出:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

解法

先设置每一行首尾元素为 1，其它元素为 0。然后根据杨辉三角，设置每一行其它元素即可。

Python3

class Solution:
    def generate(self, numRows: int) -> List[List[int]]:
        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<>();
        for (int i = 0; i < numRows; ++i) {
            List<Integer> t = new ArrayList<>();
            for (int j = 0; j < i + 1; ++j) {
                boolean firstOrLast = j == 0 || j == i;
                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;
    }
}

TypeScript

function generate(numRows: number): number[][] {
    if (numRows == 0) return [];
    let ans = [[1]];
    for (let i = 1; i < numRows; ++i) {
        ans.push(new Array(i + 1).fill(1));
        let half = i >> 1;
        for (let j = 1;j <= half; ++j) {
            let cur = ans[i - 1][j - 1] + ans[i - 1][j];
            ans[i][j] = cur;
            ans[i][i - j] = cur;
        }
    }
    return ans;
};

C++

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> res;
        for (int i = 0; i < numRows; ++i) {
            vector<int> t(i + 1);
            t[0] = 1;
            t[i] = 1;
            for (int j = 1; j < i; ++j) {
                t[j] = res[i - 1][j - 1] + res[i - 1][j];
            }
            res.push_back(t);
        }
        return res;
    }
};

Go

func generate(numRows int) [][]int {
	res := make([][]int, numRows)
	for i := 0; i < numRows; i++ {
		t := make([]int, i+1)
		t[0] = 1
		t[i] = 1
		for j := 1; j < i; j++ {
			t[j] = res[i-1][j-1] + res[i-1][j]
		}
		res[i] = 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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