Given numRows, generate the first numRows of Pascal's triangle.
For example, given numRows = 5,
Return
[
[1],
[1,1],
[1,2,1],
[1,3,3,1],
[1,4,6,4,1]
]
很常规的思想，构造从第3行开始构造每一个数组，每一个数组只构造前一半。
/**
* @param {number} numRows
* @return {number[][]}
*/
var generate = function(numRows) {
if (numRows===0) {
return [];
} else if (numRows===1) {
return [[1]];
} else if (numRows===2){
return [[1],[1,1]];
} else {
var a = [[1],[1,1]];
for (var i=2;i<numRows;i++) {
a[i]=[];
a[i][0]=1;
var tempp = i+1;
var temp = parseInt(tempp/2);
for (var j = 1;j<tempp/2;j++) {
a[i][j] = a[i-1][j-1]+a[i-1][j];
}
for (j = temp; j<tempp;j++) {
a[i][j]=a[i][tempp-1-j];
}
}
return a;
}
};
