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64 Minimum Path Sum
70 Climbing Stairs
85 Maximal Rectangle
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368 Largest Divisible Subset
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471 Encode String with Shortest Length
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552 Student Attendance Record II
562 Longest Line of Consecutive One in Matrix
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629 K Inverse Pairs Array
638 Shopping Offers
639 Decode Ways II
650 2 Keys Keyboard
651 4 Keys Keyboard
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70 Climbing Stairs

1. Question

You are climbing a stair case. It takesnsteps to reach to the top.
Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?
Note:Given n will be a positive integer.
Example 1:
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Input: 2
2
​
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Output: 2
4
​
5
Explanation:
6
There are two ways to climb to the top.
7
​
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1. 1 step + 1 step
9
2. 2 steps
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Example 2:
1
Input: 3
2
​
3
Output: 3
4
​
5
Explanation:
6
There are three ways to climb to the top.
7
​
8
1. 1 step + 1 step + 1 step
9
2. 1 step + 2 steps
10
3. 2 steps + 1 step
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2. Implementation

(1) DP
1
public class Solution {
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public int climbStairs(int n) {
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if (n <= 2) {
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return n;
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}
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​
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int[] step = new int[n + 1];
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​
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step[0] = 0;
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step[1] = 1;
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step[2] = 2;
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​
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for (int i = 3; i <= n; i++) {
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step[i] = step[i - 1] + step[i - 2];
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}
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return step[n];
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}
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}
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3. Time & Space Complexity

时间复杂度O(n), 空间复杂度O(n)
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Contents
70. Climbing Stairs
1. Question
2. Implementation
3. Time & Space Complexity