Given a non-negative integer, you could swap two digits at most once to get the maximum valued number. Return the maximum valued number you could get.

Example 1:

Input: 2736
Output: 7236
Explanation: Swap the number 2 and the number 7.

Example 2:

Input: 9973
Output: 9973
Explanation: No swap.

Note:

1. The given number is in the range [0, 108]

解法：从后向前扫，遇到比max_value 大的就记录这个最大数的值和位置，继续向前扫，遇到小于这个max_value时，就记录这个交换位置， 因为越往左扫数位越高，交换后整个数字值越大。 

Java:

class Solution {
public int maximumSwap(int num) {
char[] digits = Integer.toString(num).toCharArray();

int[] buckets = new int[10];
for (int i = 0; i < digits.length; i++) {
buckets[digits[i] - '0'] = i;
}

for (int i = 0; i < digits.length; i++) {
for (int k = 9; k > digits[i] - '0'; k--) {
if (buckets[k] > i) {
char tmp = digits[i];
digits[i] = digits[buckets[k]];
digits[buckets[k]] = tmp;
return Integer.valueOf(new String(digits));
}
}
}

return num;
}
}　　

Python:

class Solution(object):
def maximumSwap(self, num):
"""
:type num: int
:rtype: int
"""
digits = list(str(num))
left, right = 0, 0
max_idx = len(digits)-1
for i in reversed(xrange(len(digits))):
if digits[i] > digits[max_idx]:
max_idx = i
elif digits[max_idx] > digits[i]:
left, right = i, max_idx
digits[left], digits[right] = digits[right], digits[left]
return int("".join(digits))

Python:

def maximumSwap(self, num):
A = list(str(num))
ans = A[:]
for i in xrange(len(A)):
for j in xrange(i+1, len(A)):
A[i], A[j] = A[j], A[i]
if A > ans: ans = A[:]
A[i], A[j] = A[j], A[i]

return int("".join(ans))

Python:

def maximumSwap(self, num):
A = map(int, str(num))
last = {x: i for i, x in enumerate(A)}
for i, x in enumerate(A):
for d in xrange(9, x, -1):
if last.get(d, None) > i:
A[i], A[last[d]] = A[last[d]], A[i]
return int("".join(map(str, A)))
return num　

Python: wo

class Solution():
def maxSwap(self, num):
s = list(str(num))
i = 0
while i < len(s):
j = i + 1
swap = i
mx = s[i]
while j < len(s):
if s[j] > mx:
mx = s[j]
swap = j
j += 1
if swap != i:
s[i], s[swap] = s[swap], s[i]
break

i += 1

return int(''.join(s))

C++:

class Solution {
public:
int maximumSwap(int num) {
string digits = to_string(num);
int left = 0, right = 0;
int max_idx = digits.length() - 1;
for (int i = digits.length() - 1; i >= 0; --i) {
if (digits[i] > digits[max_idx]) {
max_idx = i;
} else if (digits[max_idx] > digits[i]) {
left = i;
right = max_idx;
}
}
swap(digits[left], digits[right]);
return stoi(digits);
}
};

C++:

int maximumSwap(int num) {
string numstr = std::to_string(num);

int maxidx = -1; int maxdigit = -1;
int leftidx = -1; int rightidx = -1;

for (int i = numstr.size() - 1; i >= 0; --i) {
// current digit is the largest by far
if (numstr[i] > maxdigit) {
maxdigit = numstr[i];
maxidx = i;
continue;
}

// best candidate for max swap if there is no more
// such situation on the left side
if (numstr[i] < maxdigit) {
leftidx = i;
rightidx = maxidx;
}
}

// num is already in order
if (leftidx == -1) return num;

std::swap(numstr[leftidx], numstr[rightidx]);

return std::stoi(numstr);
}

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