leetcode 对角线遍历 JavaScript

/**
 * @param {number[][]} matrix
 * @return {number[]}
 */
var findDiagonalOrder = function(matrix) {
    if(matrix == []) return []
    let m = matrix.length
    let n = matrix[0].length
    let i=0,j=0
    let arr = []
    for(let l = 0;l<m*n;l++){
        arr.push(matrix[i][j])
        if((i+j)%2==0){  
            if(j == n-1){
                i++
            }else if(i==0){
                j++
            }else{
                i--
                j++
            }
        }else{
            if(i == m-1){
                j++
            }else if(j == 0){
                i++
            }else{
                i++
                j--
            }
        }
    }
    return arr
};