62. Unique Paths

A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).

The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked 'Finish' in the diagram below).

How many possible unique paths are there?

Example 1:

Input: m = 3, n = 7
Output: 28

Example 2:

Input: m = 3, n = 2
Output: 3

Explanation: From the top-left corner, there are a total of 3 ways to reach the bottom-right corner:

1. Right -> Down -> Down
2. Down -> Down -> Right
3. Down -> Right -> Down

Example 3:

Input: m = 7, n = 3
Output: 28

Example 4:

Input: m = 3, n = 3
Output: 6

• Constraints:
  • 1 <= m, n <= 100
  • It's guaranteed that the answer will be less than or equal to 2 * 109.

Analyze

记忆递归法:

const cache = {}
/**
 * @param {number} m
 * @param {number} n
 * @return {number}
 */
var uniquePaths = function (m, n) {
  if (m === 1 || n === 1) {
    return 1
  }

  if (cache[`${m}~${n}`]) {
    return cache[`${m}~${n}`]
  } else {
    const nums = uniquePaths(m - 1, n) + uniquePaths(m, n - 1)
    cache[`${m}~${n}`] = nums

    return nums
  }
}

动态规划法:

/**
 * @param {number} m
 * @param {number} n
 * @return {number}
 */
var uniquePaths = function(m, n) {
  const cache = {
    [`${m - 1}_${n - 1}`]: 1
  }
  for (let x = m - 1; x >= 0; x--) {
    for (let y = n - 1; y >= 0; y--) {
      if (x === m - 1 && y === n - 1) {
        continue
      }
      if (y + 1 === n) {
        cache[`${x}_${y}`] = cache[`${x + 1}_${y}`]
      } else if (x + 1 === m) {
        cache[`${x}_${y}`] = cache[`${x}_${y + 1}`]
      } else {
        cache[`${x}_${y}`] = cache[`${x}_${y + 1}`] + cache[`${x + 1}_${y}`]
      }
    }
  }
  return cache['0_0']
}

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