498.js

/** ------------------------------------------------------------------------------
 * 
 * 498. Diagonal Traverse
 * Topics: Array, Matrix
 * https://leetcode.com/problems/diagonal-traverse/?envType=daily-question&envId=2025-08-25
 * 
 ------------------------------------------------------------------------------ */
/**
 * @param {number[][]} mat
 * @return {number[]}
 */
var findDiagonalOrder = function (mat) {
  let result = []
  let total = mat[0].length * mat.length - 1
  let x = 0
  let y = 0
  let slope = 1

  for (let i = 0; i < total + 1; i++) {
    result.push(mat[y][x])
    const oldX = x
    const oldY = y
    if (slope === 0) {
      x--
      y++
    } else {
      x++
      y--
    }
    if (y < 0) {
      y = 0
      slope = 0
    }
    if (x < 0) {
      x = 0
      slope = 1
    }
    if (y === mat.length) {
      y--
      x++
      if (oldX === x) {
        x++
      }
      slope = 1
    } else if (x === mat[0].length) {
      y++
      x--
      if (oldY === y) {
        y++
      }
      slope = 0
    }
  }

  return result
}

console.log(
  findDiagonalOrder([
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
  ]),
)