递归_action

import (
	"strconv"
	"strings"
)

func restoreIpAddresses(s string) []string {
	ans, n := []string{}, len(s)
	current, currentCharLen := []string{}, 0
	ok := func(start, end int) bool {
		// 101023 1.01.
		// 10101010 1.010.
		sub := s[start : end+1]
		i, _ := strconv.ParseInt(sub, 10, 0)
		left, right := func() (int64, int64) {
			l, r := -1, 10
			switch len(sub) {
			case 2:
				l, r = 9, 100
			case 3:
				l, r = 99, 256
			}
			return int64(l), int64(r)
		}()
		return i > left && i < right
	}
	var backtrack func(start int)
	backtrack = func(start int) {
		for i := start; i < n; i++ {
			for j := i; j < n; j++ {
				if len(current) < 4 && ok(i, j) {
					current = append(current, s[i:j+1])
					currentCharLen += j - i + 1
					if currentCharLen == n && len(current) == 4 {
						ans = append(ans, strings.Join(current, "."))
					}
					backtrack(j + 1)
					current = current[0 : len(current)-1]
					currentCharLen -= j - i + 1
				}
			}
		}
	}
	backtrack(0)
	return ans
}

/*
Restore IP Addresses - LeetCode https://leetcode.com/problems/restore-ip-addresses/

Example 1:

Input: s = "25525511135"
Output: ["255.255.11.135","255.255.111.35"]
Example 2:

Input: s = "0000"
Output: ["0.0.0.0"]
Example 3:

Input: s = "101023"
Output: ["1.0.10.23","1.0.102.3","10.1.0.23","10.10.2.3","101.0.2.3"]


Constraints:

1 <= s.length <= 20
s consists of digits only.
*/

练习递归的好问题 - 分割回文串 - 力扣（LeetCode） https://leetcode.cn/problems/palindrome-partitioning/solution/-by-followingu-vp10/

package backtracking

func palindromePartitioning(s string) [][]string {
	var isPalindrome = func(start, end int) bool {
		for start < end {
			if s[start] != s[end] {
				return false
			}
			start++
			end--
		}
		return true
	}
	n := len(s)
	isPalindromeReuse := make([][]bool, n)
	for i := 0; i < n; i++ {
		isPalindromeReuse[i] = make([]bool, n)
		for j := i; j < n; j++ {
			isPalindromeReuse[i][j] = isPalindrome(i, j)
		}
	}

	ans := [][]string{}
	current, currentLen := []string{}, 0
	var backtrack func(start int)
	backtrack = func(start int) {
		for i := start; i < n; i++ {
			for j := i; j < n; j++ {
				// if isPalindrome(i,j) {
				if isPalindromeReuse[i][j] {
					current = append(current, s[i:j+1])
					currentLen += j - i + 1
					if currentLen == n {
						ans = append(ans, append([]string(nil), current...))
					}
					backtrack(j + 1)
					current = current[0 : len(current)-1]
					currentLen -= j - i + 1

				}
			}
		}
	}
	backtrack(0)
	return ans
}

/*
Palindrome Partitioning - LeetCode https://leetcode.com/problems/palindrome-partitioning/

Example 1:

Input: s = "aab"
Output: [["a","a","b"],["aa","b"]]
Example 2:

Input: s = "a"
Output: [["a"]]


Constraints:

1 <= s.length <= 16
s contains only lowercase English letters.
*/

https://leetcode-cn.com/problems/binary-tree-preorder-traversal/

https://leetcode-cn.com/problems/binary-tree-inorder-traversal/

https://leetcode-cn.com/problems/binary-tree-postorder-traversal/

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func inorderTraversal(root *TreeNode) []int {
    if root == nil {
        return nil
    }
    l := []int{}
    l = append(l, inorderTraversal(root.Left)...)
    l = append(l, root.Val)
    l = append(l, inorderTraversal(root.Right)...)
    return l
}
 
func preorderTraversal(root *TreeNode) []int {
    if root == nil {
        return nil
    }
    l := []int{}
    l = append(l, root.Val)
    l = append(l, preorderTraversal(root.Left)...)
    l = append(l, preorderTraversal(root.Right)...)
    return l
}
 
func postorderTraversal(root *TreeNode) []int {
    if root == nil {
        return nil
    }
    l := []int{}
    l = append(l, postorderTraversal(root.Left)...)
    l = append(l, postorderTraversal(root.Right)...)
    l = append(l, root.Val)
    return l
}