杨辉三角本质可以看为一个以对角线为轴一少半是0的矩阵。
既然是矩阵，可以获取输入中行秩列秩中较大的进行杨辉三角的初始化，这样就会永远包裹输入。
初始化之后就是根据输入的行列进行遍历取出对应的值。
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| <!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>杨辉三角</title>
</head>
<body>
<div>
请输入行数和列数(用空格隔开)：
<input type="text" id="numInput">
</div>
<div>
<button onclick="calc()">得出结果</button>
<span id="result"></span>
</div>
<script>
let numInput = document.querySelector('#numInput');
let resultEle = document.querySelector('#result');
let returnMax = (a, b) => a > b ? a : b;
let calc = () => {
let row = parseInt(numInput.value.split(' ')[0]) || null;
let col = parseInt(numInput.value.split(' ')[1]) || null;
if (!row || !col) return alert('请输入必填项');
if (row < col) return alert('行必须大于列');
let yanghuiMat = [];
for (let a = 0; a <= returnMax(row, col); a++) {
yanghuiMat[a] = [];
for (let b = 0; b <= returnMax(row, col); b++) {
if (a === b) {
yanghuiMat[a][b] = 1
} else if (a < b) {
yanghuiMat[a][b] = 0
} else {
yanghuiMat[1][0] = 1
yanghuiMat[a][0] = 1
yanghuiMat[a][b] = yanghuiMat[a - 1][b - 1] + yanghuiMat[a - 1][b]
}
}
}
resultEle.innerText = yanghuiMat[row - 1][col - 1];
}
</script>
</body>
</html>
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