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problems/118-pascals-triangle.md

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## 题目
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* 118. 杨辉三角
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给定一个非负整数 numRows,生成「杨辉三角」的前 numRows 行。
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在「杨辉三角」中,每个数是它左上方和右上方的数的和。
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## 思路
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dp[i][j] = dp[i-1][j-1] + dp[i-1][j]
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## 代码
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```
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class Solution {
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/**
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* @param Integer $numRows
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* @return Integer[][]
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*/
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function generate($numRows) {
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/**
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1
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1 1
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1 2 1
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dp[i][j] = dp[i-1][j-1] + dp[i-1][j]
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*/
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$dp = [];
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for ($i = 0; $i < $numRows; $i++) {
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for ($j = 0; $j <= $i; $j++) {
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if ($j == 0 || $j == $i) {
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$dp[$i][$j] = 1;
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} else {
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$dp[$i][$j] = $dp[$i-1][$j-1] + $dp[$i-1][$j];
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}
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}
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}
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return $dp;
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}
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}
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```

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